145-147. Sections of second maturation spindles; element _x_ dividing in 146 and 147.

148. Telophase of second mitosis.

149. Telophase of second mitosis, showing ma.s.ses of chromatin left behind in cytoplasm.

150. Spermatid with extranuclear chromatin (_a_).

151. Similar stage; different view of spindle-remains (_s_) and of chromatin element (_x_{2}_).

152-153. Spermatid with divided spindle-substance and the corresponding double-tailed form.

154-155. Stages between 156 and 158.

156-157. Older spermatids than 151, showing spindle-remains (_s_) and centrosome (_c_).

158-160. Later stages in development of sperm-head.

161. Ripe spermatozoon.

162-168. Degenerate spermatids and spermatozoa.

[Ill.u.s.tration: STEVENS. PLATE V.

N. M. S. del.

BLATTELLA GERMANICA.]

PLATE VI.

_Tenebrio molitor._

FIGS. 169-170. Equatorial plates of spermatogonial mitosis, showing 19 large and 1 small chromosome.

171-175. Condensation stage, bouquet stage, spireme stage, and rather rare tetrad stage of young spermatocyte.

176. Bivalent chromosomes, with longitudinal split; small chromosome shown at _s_.

177. Bivalent chromosomes condensed into a close spireme.

178-179. Bivalent chromosomes separating for mitosis. The unsymmetrical pair shown in fig. 179.

180. Prophase of first maturation mitosis, showing the unsymmetrical pair and the tetrad nature of the symmetrical pairs.

181. Prophase of same mitosis, showing symmetrical and unsymmetrical pairs, as in figs. 178 and 179.

182. Metaphase, unsymmetrical pair out of the equatorial plane.

183. Tangential section of a spindle in metaphase, showing the unsymmetrical pair and one symmetrical pair.

184. Equatorial plate of same mitosis, 10 chromosomes.

184_a_. Early anaphase, showing separation of the elements of the unsymmetrical pair.

185. Later anaphase.

186. Polar plate, showing 9 large and 1 small chromosome.

187. Polar plate, showing 10 large chromosomes.

188. Condensation stage between the two maturation divisions.

189-190. Prophase of second maturation division, fig. 189 showing 10 equal dyads, and fig. 190, showing 9 equal and 1 small dyad.

191. Equatorial plate, showing 1 small chromosome and 9 large ones.

192. Equatorial plate, showing 10 large chromosomes.

193-194. Tangential sections of spindle in metakinesis.

195. Anaphase of same mitosis.

196. Polar plates of a spindle, showing in each 1 small chromosome and 9 large ones.

197. Polar plates of another spindle, 10 large chromosomes in each.

198. Young spermatid, showing isolated small chromosome.

199. Young spermatid, showing isolated large chromosome and rotation of nuclear contents.

200-202_a_, _b._ Older spermatids.

203. Sperm-heads, showing centrosome and granular chromatin.

204. Equatorial plate from dividing somatic cell of male pupa, showing 19 large and 1 small chromosome.

205-206. Daughter plates of a similar spindle, showing small chromosome in each; three of the large chromosomes missing in 206.

207. Equatorial plate of a dividing cell of follicle of a young egg, showing 20 large chromosomes.

208. Prophase of mitosis in a young oogonium, showing 20 large chromosomes in two sections, _a_ and _b_.

[Ill.u.s.tration: STEVENS. PLATE VI.

N. M. S. del.

TENEBRIO MOLITOR.]