[Ill.u.s.tration: FIG. 15.--The Five Primary Stages of Ontogeny. (After Haeckel.) 1. Monerula. 2. Cytula. 3. Morula. 4. Blastula. 5. Gastrula.]

We may take the following account[440] of the phylogeny of the human species, from the gastraea stage onwards, as typical of Haeckel's speculations on the evolution of the higher forms. The progenitors of man are, after the Gastraeada:--

1. Turbellaria.

*2. Scolecida. (Worms with a coelom, probably represented at the present day by _Balanoglossus_.) *3. Himatega. (Evolved from Scolecida by formation of dorsal nerve-tube and chorda, and resembling tailed larvae of Ascidians.) 4. Acrania. (With metameric segmentation. Including Amphioxus.) 5. Monorrhina. (Cyclostomes.) 6. Selachia.

7. Dipneusta.

8. Sozobranchia. (Amphibia with permanent gills.) 9. Sozura. (Tailed Amphibia.) *10. Protamnia.

*11. Promammalia.

12. Marsupialia.

13. Prosimiae.

14. Menocerca. (Tailed apes.) 15. Anthropoides.

16. Pithecanthropi.

17. Homines.

It will be noticed that except for the hypothetical forms (marked with an asterisk), which are themselves generalised cla.s.sificatory groups, the ancestral forms belong to long-recognised cla.s.ses. The whole course of the evolution follows well-worn systematic lines. This is typical of Haeckel's phylogenetic speculations.

A more abstractly morphological scheme of the evolution of Vertebrates is given in the _Systematic Phylogeny_ of 1895.[441] The ontogenetic and ancestral stages are arranged in parallel columns thus:--

Cytula. Cytaea (Protozoa).

Morula. Moraea (Coen.o.bium of Protozoa).

Blastula. Blastaea (_Volvocina_, etc.).

Depula (inv.a.g.i.n.ated blastula). Depaea.

Gastrula. Gastraea (cf. _Olynthus_, _Hydra_, and primitive Coelentera).

Coelomula (with one pair Coelomaea (cf. _Sagitta_, _Ascidia_, of coelom-pockets). and primitive Helminthes).

Chordula (with medullary Chordaea (_cf._ Ascidian larva and tube and chorda). larva of Amphioxus).

Spondula (with segmented Prospondylus (Primitive Vertebrate).

mesoderm).

This scheme differs from the earlier one chiefly in taking into account certain advances, notably as regards the cytology of the fertilised ovum and the true nature of the coelom, which had been made in the interval of some twenty years.

Haeckel's Gastraea theory, though it exercised a great influence upon the subsequent trend of phylogenetic speculation, was by no means universally accepted _telle quelle_. Opinions differed considerably as to the primitive mode of origin of the two-layered sac which was very generally admitted to be of constant occurrence in early embryogeny. Ray Lankester, in his paper of 1873, and more fully in 1877,[442] propounded a "Planula" theory, according to which the ancestral form of the Metazoa was a two-layered closed sac formed typically by delamination, less often by inv.a.g.i.n.ation. He denied that the inv.a.g.i.n.ation opening (which he named the blastopore) represented the primitive mouth,[443] holding that this was typically formed by an "inruptive" process at the anterior end of the planula, which led to the formation of a "stomodaeum." A similar process at the posterior end gave rise to the a.n.u.s and the "proctodaeum."

The question as to whether delamination or inv.a.g.i.n.ation was to be considered the more primitive process was discussed in detail by Balfour,[444] without, however, any very definite conclusion being reached. He held that both processes could be proved in certain cases to be purely secondary or adaptive, and that accordingly there was nothing to show that either of them reproduced the original mode of transition from the Protozoa to the ancestral two-layered Metazoa (p. 342). He by no means rejected the theory that the Gastraea, "however evolved, was a primitive form of the Metazoa," but, having regard to the great variations shown in the relation of the blastopore to mouth and a.n.u.s (pp. 340-1), he was inclined to think that if the gastrula had any ancestral characters at all, these could only be of the most general kind. Balfour's att.i.tude perhaps best represents the general consensus of opinion with regard to the Gastraea theory.

From the same origins as the Gastraea theory arose the theory of the coelom. The term dates back to Haeckel in 1872, and the observations which first led up to the theory were made by the men who supplied the foundations of the Gastraea theory--A. Aga.s.siz, Metschnikoff and Kowalevsky. But it was not Haeckel himself who enunciated the coelom theory.

It will be remembered that Remak introduced in 1855 the conception of the mesoderm as an independent layer derived from the endoderm. The pleuro-peritoneal or body-cavity was formed as a split in the "ventral plates" of the mesoderm. Haeckel's "coelom" corresponded to the "pleuro-peritoneal cavity" of Remak, but his view of the origin of the mesoderm brought him much closer to von Baer's conception of the origin of _two_ secondary layers from ectoderm and endoderm respectively than to Remak's conception of the mesoderm as a single independent layer.

Much uncertainty reigned at the time as to the exact manner of origin of the mesoderm;[445] some held that it developed from the ectoderm, others that it originated in the endoderm, while still others, and among them Haeckel, considered that part of it came from the ectoderm and part from the endoderm (pp. 23-4, 1874).

The solution of the problem came from those observations on the development of the lower forms to which we have just alluded.

The early history of these discoveries and of the theory which grew out of them has been well summarised by Lankester,[446] and may conveniently be given in his own words:--

"As far back as 1864 Alexander Aga.s.siz ("Embryology of the Star-fish,"

in _Contributions to the Natural History of the United States_, vol. v., 1864) showed in his account of the development of Echinoderma that the great body-cavity of those animals developed as a pouch-like outgrowth of the archenteron of the embryo, whilst a second outgrowth gave rise to their ambulacral system; and in 1869 Metschnikoff (_Mem. de l'Acad.

imperiale des Sciences de St Petersbourg_, series vii., vol. xiv., 1869), confirmed the observations of Aga.s.siz, and showed that in Tornaria (the larva of Balanoglossus) a similar formation of body-cavities by pouch-like outgrowths of the archenteron took place.

Metschnikoff has further the credit of having, in 1874 (_Zeitsch. wiss.

Zoologie_, vol. xxiv., p. 15, 1874), revived Leuckart's theory of the relationship of the coelenteric apparatus of the Enterocoela to the digestive ca.n.a.l and body-cavities of the higher animals. Leuckart had in 1848 maintained that the alimentary ca.n.a.l and the body-cavity of higher animals were united in one system of cavities in the Enterocoela (_Verwandschaftsverhaltnisse der wirbellosen Thiere_, Brunswick, 1848).

Metschnikoff insisted upon such a correspondence when comparing the Echinoderm larva, with its still continuous enteron and coelom, to a Ctenophor, with its permanently continuous system of cavities and ca.n.a.ls. Kowalevsky, in 1871, showed that the body-cavity of Sagitta was formed by a division of the archenteron into three parallel cavities, and in 1874 demonstrated the same fact for the Brachiopoda. In 1875 (_Quart. Journ. Micr. Sci._, vol. xv., p. 52) Huxley proposed to distinguish three kinds of body-cavity: the schizocoel, formed by the splitting of the mesoblast, as in the chick's blastoderm; the enterocoel, formed by pouching of the archenteron, as in Echinoderms, Sagitta and Brachiopoda; and the epicoel.... Immediately after this I put forward the theory of the uniformity of origin of the coelom as an enterocoel (_Quart. Journ. Micr. Sci._, April, 1875).... My theory of the coelom as an enterocoel was accepted by Balfour and was greatly strengthened by his observations on the derivation of both notochord and mesoblastic somites from archenteron in the Elasmobranchs, and by the publication in 1877 by Kowalevsky of his second paper on the development of Amphioxus--in which the actual condition which I had supposed to exist in the Vertebrata was shown to occur, namely, the formation of the mesoblast as paired pouches in which a narrow lumen exists, but is practically obliterated on the nipping-off of the pouch from the archenteron, after which process it opens out again as coelom" (pp.

16-18).

The enterocoelic theory was taken up by O. and R. Hertwig as an essential part of their _Coelomtheorie_.[447] In a lengthy series of monographs these workers made a comparative study of the mode of formation of the middle layer, and arrived at a coherent theory of its origin. They distinguished in the middle layer two quite distinct elements, the mesoblast proper, formed by the ev.a.g.i.n.ation of the walls of the archenteron, and the mesenchyme, formed by free cells budded off from the germ-layers. The following pa.s.sage gives a good idea of their views and of the phylogenetic implications involved:--"Ectoblast and entoblast are the two primary germ-layers which arise from the inv.a.g.i.n.ation of the blastula; they are always the first to be laid down, and they can be directly referred back to a simple ancestral form, the Gastraea; they form the limits of the organism towards the exterior and towards the archenteron. The parietal and visceral mesoblast, or the two middle layers, are always of later origin, and arise through ev.a.g.i.n.ation or plaiting of the entoblast, the remainder of which can now be distinguished as secondary entoblast from the primary. They form the walls of a new cavity, the enterocoel, which is to be regarded as a nipped-off diverticulum of the archenteron. Just as the two-layered animals can be derived from the Gastraea, so can the four-layered animals be derived from a Coelom form. Embryonic cells, which become singly detached from their epitheliar connections we consider to be something quite different from the germ-layers, and accordingly we call them by the special name of mesenchyme germs or primary cells of the mesenchyme.

They may develop both in two-layered and in four-layered animals. Their function is to form between the epithelial limiting layers a secreted tissue (_Secretgewebe_) or connective tissue with scattered cells, which cells can undergo, like the epithelial elements, the most varied modifications.... This secreted tissue in its simple or in its differentiated state, with all its derivatives, we call the mesenchyme"

(p. 122).

The important point for us is that, just as all Metazoa were considered by Haeckel to be descended from the Gastraea, so all Coelomati were held by the Hertwigs to be derived from an original coelomate _Urform_. In both cases an embryological archetype becomes a hypothetical ancestral form.

The Coelom theory was considerably modified, extended and developed by later workers, particularly as regards the relations to the coelom of the genital organs and ducts and the nephridia, but no special methodological interest attaches to these further developments.[448] We shall here focus attention upon one interesting line of speculation followed out in this country particularly by Sedgwick--the theory of the Actinozoan ancestry of segmented animals. Its relation to the Coelom theory lies in the fact that Sedgwick regarded the segmentation of the body as moulded upon the segmentation of the mesoblast, which in its turn, as Kowalevsky and Hatschek had shown, was a consequence of its mode of origin as a series of pouches of the archenteron. In other respects Sedgwick's speculations link on more closely to the Gastraea theory, for one of his main contentions is that the blastopore or _Urmund_ is h.o.m.ologous throughout at least the three metameric phyla. In following up Balfour's observations on the development of _Peripatus_,[449] Sedgwick was struck with the close resemblance existing between the elongated slit-like blastopore of this form (giving rise to both mouth and a.n.u.s), with its border of nervous tissue, and the slit-like mouth of the Actinozoan (functioning both as mouth and a.n.u.s), round which, as the Hertwigs had shown, there lies a special concentration of nerve cells and nerve fibres. He found another point of resemblance in the gastric pouches of the Actinozoa, which he h.o.m.ologised directly with the enterocoelic pouches of the Coelomati. He was led to enunciate the following theses:--[450] (1) that the mouth and a.n.u.s of Vermes, Mollusca, Arthopoda, and probably Vertebrata, is derived from the elongated mouth of an ancestor resembling the Actinozoa; (2) that somites are derived from a series of archenteric pouches, like those of Actinozoa and Medusae; (3) that excretory organs (nephridia, segmental organs) are derived from parts of these pouches which in the ancestral form, as in many polyps, were connected by a circular or longitudinal ca.n.a.l, and opened to the exterior by pores. This longitudinal ca.n.a.l was lost in Invertebrates, but persisted in Vertebrates as the p.r.o.nephric duct, while the pores remained in Invertebrates and disappeared in Vertebrates; (4) that the tracheae of Arthropods, as well as the ca.n.a.l of the central nervous system in Vertebrates, are to be traced back to certain ectodermal pits in the diploblastic ancestor comparable to the sub-genital pits of the Scyphomedusae. These ectodermal pits were all originally respiratory organs. "The essence of all these propositions," he writes, "lies in the fact that the segmented animals are traced back not to a triploblastic unsegmented ancestor, but to a two-layered Coelenterate-like animal with a pouched gut, the pouching having arisen as a result of the necessity for an increase in the extent of the vegetative surfaces in a rapidly enlarging animal (for circulation and respiration)" (p. 47). "I have attempted to show," he writes further on, "that the majority of the Triploblastica ... are built upon a common plan, and that that plan is revealed by a careful examination of the anatomy of Coelenterata; that all the most important organ-systems of these Triploblastica are found in a rudimentary condition in the Coelenterata; and that all the Triploblastica referred to must be traced back to a diploblastic ancestor common to them and the Coelenterata" (p. 68). The main a.s.sumption was that the neural or blastoporal surface must be h.o.m.ologous throughout the Metazoa, though it was dorsal in the Chordata, ventral in the Annelida and Arthropoda. He derived the central nervous system of the Chordata from the circ.u.moral ring of the common ancestor by means of the hypothesis that both the pre-blastoporal and the post-blastoporal parts of it disappeared.[451]

The characteristic relation of the central nervous system to the blastopore in Annelida and Vertebrates had already been pointed out by Kowalevsky,[452] who had also sketched a theory of the common descent of these two phyla from an ancestral form in which the nervous system encircled the blastopore.

In 1882, before the publication of Sedgwick's papers, A. Lang[453] had put forward the somewhat similar view that the stomach-diverticula of the Turbellaria, which he had found to be segmentally arranged in certain Triclads, were the morphological equivalents of the enterocoelic pouches of higher animals. This view, however, he soon gave up.[454] Sedgwick's views found a supporter in A. A. W. Hubrecht,[455] who utilised them in connection both with his speculations on the relation of Nemertines to Vertebrates, and with his exhaustive work on the early development of the Mammalia. He postulated as the far-back ancestor of Vertebrates, "an actinia-like, vermiform being, elongated in the direction of the mouth-slit" (p. 410, 1906), and derived the central nervous system from the circ.u.m-oral ring of this primitive form, the notochord from its stomodaeum, and the coelom from the peripheral parts of the gastric cavity (p. 169, 1909).

[424] Gegenbaur, _Zeits. f. wiss. Zool._, v., 1853.

[425] Remak, _loc. cit._, p. 183, pl. xii.

[426] Lereboullet, _Ann. Sci. nat._ (4) xviii., pp. 118-9, 1862.

[527] Lereboullet, in Remak, p. 183 f.n.

[428] Kowalevsky, _Mem. Acad. Sci. St Petersbourg_ (Petrograd), (7), x. and xi., 1866 and 1867.

[429] A. Aga.s.siz, _Contrib. Nat. Hist. United States_, v., 1864.

[430] _Mem. Acad. Sci. St Petersbourg_ (Petrograd), (7), xiv., 1869.

[431] "Embryolog. Studien an Wurmern u. Arthropoden,"

_Mem. Acad. Sci. St Petersbourg_ (Petrograd), (7), xvi., 1870.

[432] _Die Kalkschwamme_, 3 vols., Berlin, 1872. General chapters translated in _Ann. Mag. Nat. Hist._ (4), xi., pp. 241-62, 421-30, 1873.

[433] "Die Gastraea-Theorie, die phylogenetische Cla.s.sification des Thierreichs und die h.o.m.ologie der Keimblatter." _Jenaische Zeitschrift_, viii., pp. 1-55, 1874. "Die Gastrula und die Eifurchung der Thiere,"

_ibid._, ix., pp. 402-508, 1875. "Die Physemarien, Gastraeaden der Gegenwart," and "Nachtrage zur Gastraea-Theorie," _ibid._, x., pp. 55-98, 1876.

Republished in _Biologische Studien_, 2nd part, _Studien zur Gastraea-Theorie_, 270 pp., 14 pls., Jena, 1877.

[434] See _Ann. Mag. Nat. Hist._ (4), xi., p. 253.

[435] Term first introduced in _Die Kalkschwamme_, p. 468, 1872.

[436] "On the Primitive Cell-layers of the Embryo as the Basis of Genealogical Cla.s.sification of Animals, and on the Origin of Vascular and Lymph Systems," _Ann. Mag.

Nat. Hist._ (4), xi., pp. 321-38, 1873.

[437] First distinguished in _Die Kalkschwamme_, i., p.

465.

[438] Even in the 'seventies it was still believed by many that the egg-nucleus disappeared on fertilisation. The true nature of the process was not fully made out till 1875, when O. Hertwig observed the fusion of egg- and sperm-nuclei in _Toxopneustes (Morph. Jahrb._, i., 1876).