Cuvier was the critic of Geoffroy and the transcendentalists, of Lamarck and the believers in the _ech.e.l.le des etres_, evolutionary or ideal. Von Baer also, though influenced greatly by _Naturphilosophie_, turned against the exaggerations of the transcendental school, and by his unanswerable criticism of the theory of parallelism took away the ground from those who too easily believed in an historical evolution.[178]

We have seen what were von Baer's criticisms of the theory of parallelism. If we turn to the later writings of Cuvier we find the essential criticism expressed in similar terms. Speaking of an attempt which had been made to show that fish were molluscs developed to a higher degree, he wrote in 1828,[179] "Let us draw the conclusion that even if these animals can be spoken of as enn.o.bled molluscs, as molluscs raised to a higher power, or if they are embryos of reptiles, the beginnings of reptiles, this can be true of them only in an abstract and metaphysical sense, and that even this abstract statement would be very far from giving an accurate idea of their organisation." From the fact that the respiratory and circulatory organs of fish greatly resemble those of tadpoles the conclusion has been drawn that fish are in a sense embryos of Amphibia (p. 547). But this manner of viewing things is none the less vicious, "for this reason ... that it considers only one or two points and neglects all the others" (p. 548), and is directly contrary to common sense. There is never a recapitulation of total organisations, only at the most of single organs.

It will be remembered that Cuvier opposed and demolished the theory of the _ech.e.l.le des etres_, not only by showing that there were in Nature four entirely different plans of animal structure, but also by demonstrating that even the animals of each single _Embranchement_ could not readily be arranged in one series, that a serial arrangement was really valid only for their separate organs. Von Baer also held that there are four distinct types of structure; he, too, combated the idea of gradation within the limits of the type. In so far as species represent successive stages in the development, the _Ausbildung_, of the type, so far can the idea of a scale of beings be applied. But the members of a type follow not one line of evolution but several diverging lines, in direct adaptation to different environmental conditions, so that a serial arrangement of them is not as a rule possible. It may be possible to establish a serial arrangement of single organs from the simplest to the most complex. But each organ or organ-system will require a different serial arrangement, for the different systems vary on different lines and an animal may be highly developed in respect of one system and little developed in respect of all the others. Man, for instance, is the highest animal only in respect of his nervous system.

The idea of the scale of beings has therefore only a very limited application even within the limits of the type. Applied to the whole animal kingdom it becomes merely absurd.

Another point of resemblance between Cuvier and von Baer was that Cuvier, though essentially a student of adult structure, did recognise the importance of embryology; following up some observations of Dutrochet he studied the foetal membrane of mammals and tried to establish their h.o.m.ologies.[180] And in his criticism of the vertebral theory of the skull he advanced as an argument against the basisphenoid being a vertebral centrum the fact (established by Kerkring, 1670), that it develops from two centres.[181] Von Baer's relation to transcendental anatomy was in some ways a close one, though he was a trenchant critic of the extreme views of the school.[182] He took from Oken the idea that a simple fundamental plan rules the organisation of all Vertebrates; "That jaws and limbs are modifications of one fundamental form is readily apparent, and, after Oken, the fact ought to be accepted by the majority of those naturalists who do not refuse to admit the existence of a general type from which the diversity of structure is developed" (i., p. 192). He accepted the vertebral theory of the skull in its main lines, and used his embryological knowledge to support the idea that jaws correspond to limbs--the latter point as part of the transcendental idea that the hind end of the body repeats the organisation of the anterior part (i., p. 192). The particular form which his theory of the relation of jaws to limbs took is shown in the following pa.s.sage:--"The maxillary bone has ... the significance of an extremity and at the same time that of a rib or lower arch of a vertebra, just as the pelvic bones unite in themselves the signification of ribs and proximal members of the hinder extremity" (Meckel's _Archiv_, p. 367, 1826).

He appreciated the morphological idea of the serial repet.i.tion of parts, and gave it accurate formulation. The whole vertebrate body, he considered, was composed of a longitudinal series of _morphological elements_, each of which was made up a section from each of the fundamental organs--a vertebra, a section of the nerve-cord, and so on (_Entwickelungsgeschichte_, ii., p. 53). Groups of these morphological elements formed _morphological divisions_, such as the vertebral segments of the head with their highly developed neural arches, or the segments of the neck with their undeveloped haemal arches. The morphological elements are clearly shown only in the animal parts, but there are indications in the embryo of a segmentation also of the vegetative parts,--the gill-slits, for instance, and the vascular arches. The vegetative parts, however, develop on the whole unsymmetrically (_cf._ Bichat). These elements which von Baer distinguishes are morphological units, as he himself points out, contrasting them with organs which are not usually units in a morphological sense. "We call organ," he writes, "each part that has by reason of its form or its function a certain distinctiveness, but this concept is very indefinite, and possesses, from a morphological point of view, little value. For this reason it seems necessary to introduce into scientific morphology the concepts of morphological elements and divisions" (ii., p. 84).

Von Baer exercised a very considerable influence upon the subsequent trend of morphological theory. By his criticism of the Meckel-Serres theory, he rid morphology for a time of an idea which was leading it astray; by his subst.i.tution of the law that development is always from the general to the special, he set morphologists looking for the archetype in the embryo, not in the adult alone, and made them realise that h.o.m.ologies could often best be sought in the earliest stages of development; by formulating the germ-layer theory he supplied morphologists with a new criterion of h.o.m.ology, based upon the special relations of the parts (germ-layers) which are first differentiated in all development. He made the study of development an essential part of morphology.

[166] _De generatione Animalium_.

[167] _De formato foetu_, ? 1600; _De formatione foetus_, 1604.

[168] _Exercitationes de generatione animalium_, 1651.

[169] _De formatione pulli in ovo_, 1673; _De ovo incubato_, 1686.

[170] _De formatione pulli in ovo_, 1757-8; _Sur la formation du coeur dans le poulet_, 1758.

[171] _Theoria generatioinis_, 1759; _De formatione intestinorum_, 1768-9.

[172] _Beitrage zur Entwickelung des Huhnchens im Ei._ Wurzburg, 1818. Also in Latin in shorter form, 1817.

[173] _Untersuchungen u. die Entwickelungsgeschichte der Fische_; Leipzig, 1835.

[174] Cuvier, in 1812, _Ann. Mus. d'Hist. Nat._, xix.; von Baer in 1816, _Nova Acta Acad. Nat. Cur._ See _Entwickelungsgeschichte der Thiere_, i., p. vii., f.n.

[175] Compare a parallel pa.s.sage in Prevost et Dumas:--"At the very first sight one will be struck with the resemblance between the forms of the very early embryos of these two cla.s.ses, a resemblance so extraordinary that one cannot refuse to admit the conclusions resulting from it. The resemblance is so striking that one can defy the most experienced observer to distinguish in any way the embryos of dog or rabbit ...

from those of fowls or ducks of a corresponding age."--_Ann. Sci. nat._, iii., p. 132, 1824.

[176] _De l'organisation des Animaux_, i., p. 140, 1822.

[177] "Ueber das aussere und innere Skelet," Meckel's _Archiv fur Anat. u. Physiol._, pp. 327-76, 1826. See, too, his _Entwickelungsgeschichte_, i., pp. 181, ff.

[178] Von Baer wrote an appreciative biography of Cuvier, published posthumously in 1897, _Lebensgeschichte Cuviers_, ed. L. Stieda. French trans. in _Ann. Sci.

Nat._ (_Zool._), ix., 1907.

[179] Cuvier et Valenciennes, _Histoire naturelle des Poissons_, i., p. 550.

[180] _Mem. Mus. d'Hist. Nat._, iii., pp. 98-119, 1817.

[181] _Lecons d'Anatomie comparee_, 3rd ed., vol. i., p.

414, Bruxelles, 1836.

[182] In the aforementioned paper in Muller's _Archiv_ he criticises Carus vigorously and is sarcastic on Geoffroy.

CHAPTER X

THE EMBRYOLOGICAL CRITERION

Pander's work of 1817 was the forerunner of an embryological period in which men's hopes and interest centred round the study of development.

"With bewilderment we saw ourselves transported to the strange soil of a new world," wrote Pander, and many shared his hopeful enthusiasm. K. E.

von Baer's _Entwickelungsgeschichte_ was by far the greatest product of this time, but it stands in a measure apart; we have in this chapter to consider the lesser men who were Baer's contemporaries, friends, followers or critics.

It was largely a German science, this new embryology, and its leaders were all personally acquainted. Pander, von Baer and Rathke were on friendly terms with one another; von Baer dedicated his master-work to Pander; Rathke dedicated the second volume of his _Abhandlungen_ to von Baer. Interest in the new science was, however, not confined to Germany.

In Italy, Rusconi commenced in 1817 his pioneer researches on the development of the Amphibia with a _Descrizione anatomica degli organi della circolazione delle larve delle Salamandre aquatiche_ (Pavia), in which he traced the metamorphoses of the aortic arches. This was followed in 1822 by his _Amours des Salamandres aquatiques_ (Milan), and in 1826 by his memoir _Du developpement de la grenouille_ (Milan). In this last paper he described how the dark upper hemisphere of the frog's egg grows down over the lower white hemisphere and leaves free only the yolk plug; he observed the segmentation cavity and the archenteron, but thought that the former became the alimentary ca.n.a.l; he observed and interpreted rightly the formation of the medullary folds. The circular blastopore in the frog in later years often went by the name of the a.n.u.s of Rusconi.

In France Dutrochet[183] investigated the foetal membranes in various vertebrate cla.s.ses; Prevost and Dumas studied the very earliest stages of development in birds, mammals and amphibia (_Ann. Sci. nat._, ii., iii., 1824, xii., 1827).

A little later came Duges' studies of the osteology and myology of developing amphibia (1834),[184] and Coste's careful researches into the early developmental history of mammals.[185]

[Ill.u.s.tration: FIG. 8.--Gill-slits of the Pig Embryo. (After Rathke.)]

It was in 1825 that Heinrich Rathke (1793-1860), published his famous discovery of gill-slits in the embryo of a mammal,[186] a discovery which aroused considerable interest, and greatly stimulated embryological research. He describes how in a young embryo of a pig he saw four slits in the region of the neck, going right through into the oesophagus. They were separated by part.i.tions which he called _Kiemenbogen_ (gill-arches), and immediately in front of the first gill-slit lay the developing lower jaw. He compared these gill-slits with those of a dogfish. We reproduce his drawing of the pig-embryo (_Isis_, Pl. IV., fig. 1).

Later in the same year Rathke discovered gill-slits in the chick,[187] in this case finding only three. He described growing out from in front of the first slit a structure which he compared to the operculum or gill-cover of a fish.

These discoveries were confirmed and extended for the chick[188] by the embryologist Huschke, a pupil of Oken. Like Rathke, he found only three indubitable gill-slits, but he noticed that the body-wall in front of the first gill-slit was really composed of two arches, which were on the whole similar to the gill-arches. The hinder of these two seemed to him to be a horn of the hyoid, the front one, which was bent at an angle, to be the rudiment of the upper and lower jaws (p. 401). Between these two arches he found an opening, just as between two gill-arches a gill-slit.

This opening led into the mouth-cavity, and according to Huschke it became the external ear-pa.s.sage. He discovered also three pairs of aortic arches in close relation with the gill-arches, so close indeed, that he did not hesitate to call them gill-arteries, and to recognise their resemblance with the aortic arches of fish. He traced, in part at least, the metamorphosis which these aortic arches undergo. This part of his discovery he developed in fuller detail in a paper of 1828,[189] in which he gave some excellent figures.

Shortly after Huschke's first paper, von Baer published his views and observations on this subject in a short memoir in Meckel's _Archiv_.[190]

In this paper he confirmed Rathke's discovery, and described the slits and arches in the dog and the chick. Both Rathke and he found gill-slits in the human embryo about this time (p. 557). There were generally present, he found, four gill-slits, and, as Rathke had suggested, the first gill-arch became the lower jaw. Von Baer also confirmed Rathke's discovery of the operculum, a.s.signing it, however, to the second gill-arch. He refused to accept Huschke's derivation of the auditory meatus from the first gill-slit. Von Baer saw what had escaped Rathke and Huschke, that there were, not three nor four, but as many as five aortic arches.

In his view of the metamorphosis of the aortic arches in the chick the first two pairs disappeared completely, the third pair gave rise to the arteries of the head and the fore-limbs, the right side of the fourth arch became the aorta, the left half of the fourth and the right half of the fifth arch became the pulmonary arteries, while the left half of the fifth arch disappeared. This schema, which for the last three arches was the same as Huschke's, von Baer upheld for the chick even in the second volume of his _Entwickelungsgeschichte_ (p. 116); he rectified it, however, for mammals in the same volume (p. 212), deriving both pulmonary arteries from the fifth arch, and the aorta from the fourth left. He fully recognised the great a.n.a.logy of the embryonic arrangement of gill-arches and gill-arteries in Tetrapoda with their arrangement in fish (i., pp. 53, 73).

Huschke, in a paper of 1832,[191] chiefly devoted to the development of the eye, figured and described the developing upper and lower jaws, and maintained against von Baer that the first slit turns into the auditory meatus and the Eustachian tube.

These were the first papers of the embryological period. Before going on to discuss the principles which guided embryological research during the next ten or twenty years it is convenient to note what were the main lines of work characterising the period.

The typical figure of the period is Rathke, who produced a great deal of first-cla.s.s embryological work. He was, even more than von Baer, a comparative embryologist, and there were few groups of animals that he did not study. His first large publication, the _Beitrage zur Geschichte der Thierwelt_ (i.-iv., Halle, 1820-27), contained much anatomical work in addition to the purely embryological; he commenced here his series of papers on the development of the genital and urinary organs, continued in the _Abhandlungen zur Bildungsund Entwickelungs-Geschichte des Menschen und der Thiere_ (i., ii., Leipzig, 1832-3). A fellow-worker in this line was Johannes Muller, whose _Bildungsgeschichte der Genitalien_ (Dusseldorf) appeared in 1830.

In a memoir on the development of the crayfish which appeared in 1829,[192] Rathke found in an Invertebrate confirmation of the germ-layer theory propounded by Pander and von Baer. He was greatly struck by the inverted position of the embryo with respect to the yolk. In following out the development of the appendages he noticed how much alike were jaws and legs in their earliest stage, and how this supported Savigny's contention that the limbs of Arthropods belonged to one single type of structure. In his paper (1832) on the development of the fresh-water Isopod, _Asellus_,[193] Rathke returns to this point. Commenting on the original similarity in development of antennae, jaws and legs, he writes, "Whatever the doubts one may have reserved as to the intimate relation existing between the jaws and feet of articulate animals after the researches of Savigny on this subject and mine on developing crayfish, they must all fall to the ground when one examines with care the development of the fresh-water Asellus" (p. 147 of French translation).

Further comparative work by Rathke is found in the two volumes of _Abhandlungen_ and in a book, _Zur Morphologie, Reisebemerkungen aus Taurien_ (1837), which contains embryological studies of many different types, including a study of the uniform plan of arthropod limbs. Later on Rathke devoted himself more to vertebrate embryology, producing among other works his cla.s.sical papers on the development of the adder (1839), of the tortoise (1848), and of the crocodile (1866). He laid the foundations of all subsequent knowledge of the development of the blood-vascular system in a series of papers of various dates from 1838 to 1856. The diagrams in his paper on the aortic arches of reptiles (1856) were for long copied in every text-book.

Rathke was a foremost worker in another important line of embryological work, the study of the development of the skeleton and particularly of the skull. We shall discuss the history of the embryological study of the skull in some detail below; meantime, we note the two other important lines of research which characterise this period. One is the intensive study of the development of the human embryo, a study pursued by, among others, Pockels, Seiler, Breschet, Velpeau, Bischoff, Weber, Muller, and Wharton Jones.[194] The other important line--the early development of the Mammalia--was worked chiefly by Valentin,[195]

Coste,[196] and, above all, by Bischoff, whose series of papers[197] was justly recognised as cla.s.sical.

What interests us chiefly in the work of this embryological period is, of course, the relation of embryology to comparative anatomy and to pure morphology. The embryologists were not slow to see that their work threw much light upon questions of h.o.m.ology, and upon the problem of the unity of plan. Von Baer, we have seen, recognised this clearly in 1828; Rathke, in one of his most brilliant papers, the _Anatomisch-philosophische Untersuchungen uber den Kiemenapparat und das Zungenbein_ (Riga and Dorpat, 1832), used the facts of development with great effect to show the h.o.m.ology of the gill-arches and hyoid throughout the vertebrate series; Johannes Muller made great use of embryology in his cla.s.sical _Vergleichende Anatomie der Myxinoiden_ (i.

Theil, 1836), and, according to his pupil Reichert, firmly held the opinion that embryology was the final court of appeal in disputed points of comparative anatomy;[198] Reichert himself in a book of 1838 (_Vergleichende Entwickelungsgeschichte des Kopfes der nackten Amphibien_) discussed the two different methods of arriving at the "Type"--the anatomical method of comparing adults, and the embryological method of comparing embryogenies. Of the embryological method, he says, "Its aim is to distinguish during the formation of the organism the originally given, the essence of the type, and to cla.s.sify and interpret what is added or altered in the further course of development.

Embryologists watch the gradual building up of the organism from its foundations, and distinguish the fundament, the primordial form, the type, from the individual developments; they reach thus, following Nature in a certain measure, the essential structure of the organism, and demonstrate the laws that manifest themselves during embryogeny" (p.

vi.). The embryologists, influenced in this greatly by von Baer, gradually felt their way to subst.i.tuting for the "Archetype" of pure morphology what one may perhaps best call the _embryological archetype_.