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_.

[Illustration: STEVENS. PLATE VI.

N. M. S. del.

TENEBRIO MOLITOR.]

PLATE VII.

_Aphis oenotherae._

FIG. 209. Spermatogonium.

210-211. Spermatogonia in prophase of mitosis.

212. Young spermatocyte of first order.

213. Spermatocytes of first order; conjugation of the chromosomes.

214. Condensation of chromatin--spermatocytes of first order immediately
before mit