PLATE XL 



Eggs of Ciona intestinalis ; Ovocyte to Fourth Cleavage. 



Figs. 168-179 actual section*; figs. 180-187 whole eggs. Crescent substance and clear protoplasm 

 stippled. Polar bodies are actually present where drawn. 



Fig. 168. Half grown ovarian egg, showing test cells within the egg cytoplasm, which is composed of 



a layer of yolk surrounding the nucleus and a peripheral layer of clear protoplasm ; 



around the whole egg is a layer of small follicle cells which later develop into the very 



large conical cells which surround the chorion. 

 Fig. 169. Older ovarian egg, showing the area of the yolk increased and the test cells limited to the 



peripheral layer of protoplasm, which is sharply differentiated from the yolk. 

 Fig. 170. Still older ovarian egg, showing the test cells arranged in groups or "nests" at the periphery 



of the egg. 

 Fig. 171. Ripe ovarian egg ready to escape from the ovary, showing the extrusion of the test cells and 



the formation of a thick homogeneous membrane around the egg, which ultimately 



becomes the chorion. 

 Fig. 172. Free hut unfertilized egg which lias shrunken away from the chorion, showing the first 



maturation spindle, the peripheral layer of protoplasm collected over the lower pole of 



the egg and the achromatic substance of the germinal vesicle (kp) spread in a broad 



layer over the upper pole and around the central yolk. 

 Fig. 173. Extrusion of first polar body and entrance of spermatozoon ; the sperm nucleus lies in the 



peripheral layer surrounded by clear protoplasm in which astral rays are developing. 

 Fig. 174. Nearly equatorial section of the egg showing sperm nucleus and amphiaster at the posterior 



side of the egg; at the surface is the granular protoplasm of the peripheral layer. 

 Fig. 175. Section of egg in plane of first cleavage (median plane), showing the approach of the germ 



nuclei and the movement of the peripheral layer of protoplasm from the lower pole to 



the posterior side to form the crescent ; in the darkly staining substance of the crescent 



is a clear triangular area which corresponds to the clear area surrounding the sperm 



nucleus in fig. 173. 

 Fig. 176. Section of slightly older stage in plane of first cleavage, showing the union of the germ 



nuclei ; polar body out of the plane of section. 

 Fig. 177. Anaphase of first cleavage, showing the complete separateness of the nuclear and astral 



portions of the mitotic figure; the crescent substance shows at the ends of the spindle. 

 Fig, 178. Telophase of first cleavage, showing " zwischenkorper " (z), also the bending of spindle axis 



and shifting of cytoplasm and nuclei toward the animal pole. 

 Fig. 179. Anaphase of second cleavage, showing separateness of nuclear and astral portions of mitotic 



figure, also position of crescent on postero- dorsal side ; polar body out of the plane of 



section. 

 Fig. 180. Entire egg of Ciona, showing formation of second polar spindle, peripheral layer of proto- 

 plasm, sperm nucleus and aster. 

 Fig. 181. Anaphase of first cleavage viewed from posterior pole. 

 Fig. 182. Two-cell stage seen from posterior pole, cytoplasm and nuclei lie near animal pole, crescent 



near vegetal, 

 fig. 183. Four-cell -stage seen from vegetal pole ; the crescent covers about one half the surface of the 



two posterior blastomeres. 

 Fig. 1S4. Eight cell-stage, left side, showing cap of deeply staining protoplasm at posterior pole of 



cells (B 4-1 ), which later goes into the posterior mesenchyme cells (B 7 ' J , fig. 200.) 

 Fig. 185. Eight cells, posterior view; spindles of the fourth cleavage present. 

 Fig. 186. Fourth cleavage, vegetal pole. 

 Fig. 187. Fourth cleavage, animal pole; 16 cell.-. 





