84. Older spermatid, showing centrosome (_c_), axial fiber of tail, and spindle-remains (_s_). 85. Spermatid, showing acrosome material (_a_) migrating to side of nucleus opposite centrosome. 86. Slightly older spermatid. 87. Later stage of spermatid, showing condensed chromatin, elongated centrosome (_c_), acrosome material (_a_), and spindle-remains (_s_). 88-89. Older spermatids, showing formation of acrosome (_a_) and middle piece (_m_). 90-92. More advanced stages. 93. Mature spermatozoon. _Blattella germanica._ FIG. 94. Somatic cell from egg follicle, 23 chromosomes. 95. Spermatogonium, showing chromatin element (_x_) associated with a nucleolus (_n_). 96. Same, prophase of mitosis. 97. Equatorial plate of spermatogonial mitosis, 23 chromatin elements. 98. Young spermatocyte, showing centrosome (_c_) and U-shaped element (_x_). 99. Young spermatocyte, element _x_ attached to one end of a long, fine spireme. 100. Coarser spireme stage. 101-103. Bouquet stage. 104-105. Later spireme stage. 106. Various forms assumed by the combined nucleolus and element _x_; last figure from a giant cell. 107. Segmenting spireme. 108. Similar stage 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_).