tage to fig. 107, one chromosome longitudinally split;
element _x_ present.

[Illustration: STEVENS. PLATE III.

N. M. S. del.]

PLATE IV.

_Blattella germanica._

FIG. 109. Similar stage to figs. 107 and 108; chromosomes U-shaped and
not longitudinally split; two centrosomes present (_c_).

110. Longitudinally split chromosomes.

111-113. Various stages in formation of cross-shaped tetrads.

114-117. Bent rods, U-shapes, split rings, pairs of rods, and rod-shaped
tetrads (116), which are equivalent to the crosses of figs. 112-113.

118-122. Metaphase of first maturation division, showing the element _x_
in various positions.

123-127. First maturation spindle in metaphase.

128. Same in anaphase.

129-132. Late anaphase, showing element _x_ double in 129, and a lagging
tetrad in 130.

133. Telophase, with the element _x_ in one daughter cell.

134-136. Prophase of second maturation mitosis, showing dyads and
element _x_.

[Illustration: STEVENS. PLATE IV.

N. M. S. del.

BLATTELLA GERMANICA.]

PLATE V.

_Blattella germanica._

FIGS. 137-141. Dyads contracting for second maturation mitosis.

142. Equatorial plate of second maturation spindle, containing 11
chromosomes.

143-144. Same, with 11 chromosomes and the element _x_.

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 masses 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.

[Illustration: 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