Chapter 3

40. Same in anaphase.

41. Four spermatid nuclei in one cell, each nucleus containing one nucleolus.

42. A later stage, showing elongation of nuclei, centrosome and sphere at posterior end.

43-45. Later stages in the development of the spermatozoa, nucleolus grows gradually smaller.

PLATE II.

_Stenopelmatus._

FIGS. 46-47. Nuclei of spermatogonia, showing 2 and 3 nucleoli (_n_).

48-49. Prophase of spermatogonial mitosis, showing two exceptionally large chromosomes of equal length.

50. Equatorial plate of spermatogonial mitosis, 46 chromosomes.

51-54. Spermatocytes in spireme stage, nucleus containing a nucleolus (_n_), and a chromatin element (_x_), which is attached to one end of spireme and gradually increases in size during growth stage of spermatocytes.

55. Spireme longitudinally split and showing the beginning of cross formation.

56. Spireme segmented, tetrads forming.

57. One split segment and a part of another connected by bands of linin.

58. More open cross and diamond forms; element _x_ conspicuous.

59-60. More contracted cross and diamond-shaped tetrads; linin bands shown in 60, where element _x_ is also present.

61. Different forms assumed by element _x_ during tetrad stage (figs. 56-60).

62-63. Diamond-shaped and contracted cross-shaped tetrads from metaphase of first maturation mitosis, showing linin connections.

64. Diamond-shaped tetrad with spindle-fibers attached; _a-a_, probably halves of one univalent chromosome; _b-b_, halves of the other.

65. Dyad from anaphase of first maturation mitosis.

66-67. Metaphase of first maturation spindle, showing element _x_ in different positions.

68. Late anaphase of same.

69-70. Equatorial plate of first maturation spindle, 23 chromosomes and element _x_ below, in fig. 69.

71. Chromatin massed at poles of spindle; element _x_ isolated at one pole.

72-73. Two resting spermatocytes of the second order, one containing element _x_, the other not.

74-76. Successive stages of breaking down of element _x_.

77. Prophase of second division; dyads evident, but no sign of _x_ in this or following stages.

78. Second spermatocyte division--metakinesis.

79. Same; late anaphase.

PLATE III.

_Stenopelmatus._

FIG. 80. Telophase of second maturation mitosis.

81. Young spermatid, showing spindle-remains at _s_.

82. Spermatid showing a conspicuous chromatin element in nucleus, and spindle-remains (_s_) elongated.

83. Spermatid, showing centrosome (_c_) and divided spindle-remains (_s_ and _a_).

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 spermatozoön.

_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.

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_.

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 spermatozoön.

162-168. Degenerate spermatids and spermatozoa.

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 oögonium, showing 20 large chromosomes in two sections, _a_ and _b_.

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 mitosis.

215. Equatorial plate of first maturation division.

216. Same, side view, showing chromosomes double.

217-218. Anaphase of same mitosis.

219. Daughter spermatocytes of second order.

220. Equatorial plate of second maturation mitosis.

221. Anaphase of same.

222. Young spermatid.

223. Equatorial plate of first polar spindle of winter egg.

224. Equatorial plate of polar spindle of parthenogenetic egg.

225. Equatorial plate of segmentation spindle of parthenogenetic egg.

_Sagitta bipunctata._

FIG. 226. Resting spermatogonia.

227. Prophase of spermatogonial mitosis.

228. Last spermatogonial mitosis, metakinesis.

229. Anaphase of same, showing synapsis of chromosomes at pole of spindle, and element _x_.

230. Resting spermatocyte of first order.

231. Bouquet stage.

232. Later growth stage.

233. Prophase of first maturation mitosis, some of the chromosomes split longitudinally.

234. Later stage, chromosomes condensing and element _x_ dividing.

235-237. First maturation mitosis.

238. Division of element _x_ between the two maturation divisions.

239. Second maturation mitosis.

240. Anaphase of same, showing the element _x_ more deeply stained than the chromosomes.

241. Young spermatids; element _x_ still conspicuous.