PLATE XII. 



Whole Eggs of Ciona intestinalis ; Sixteen Cells to Gastrula. 



Crescent substance stippled in nil the figures; clear protoplasm also stippled in figs. 188-191. 

 Pobty bodies are actually present where drawn. 



Fig. 188. Sixteen cell stage, anterior pole. 



Fig. 189. Sixteen cells seen from vegetal pole; all but two of the cells are passing from the 5th to the 

 6th generation. 



Fig. 190. Similar stage viewed from animal pole. 



Fig. 191. Animal pole view, 5th to the 6th generation; 20 cells; when the divisions are completed, 

 which are here indicated by spindles, there will be 32 cells. 



Figs. 192-195. One and the same egg in the 32-cell-stage. Fig. 192. From animal pole. Fig. 193. 

 From vegetal pole, spindles present in many of the cells, which are about to pass into the 

 7th generation. Fig. 194. From posterior pole; all nuclei at the animal pole in a 

 resting condition. Fig. 195. Antero-vegetal view. 



Figs. 196 and 197. One and the same egg seen from the animal (ventral) and from the vegetal (dor- 

 sal) poles; most of the cells at the vegetal pole have just passed into the 7th generation ; 

 spindles are present in all the cells at the animal pole preparatory to their division into 

 the 7th generation ; 44 cells, 20 of the 6th and 24 of the 7th generation. 



Figs. 198-204. With the exception of fig. 199, all these figures represent views of one and the same 

 egg. Fig. 198. View from animal pole showing all the ectoderm cells in process of 

 division. Fig. 199. Another egg of the same stage as the preceding, but seen from a 

 more anterior point of view. Fig. 200. Vegetal pole showing all the cells at this pole 

 in a resting condition. Fig. 201. Same egg seen from a more anterior point of view. 

 Fig. 202. Same egg viewed from the anterior pole. Fig. 203. Viewed from the right 

 side showing in dotted outline the great elongation of the cells of the vegetal pole and a 

 corresponding flattening of the cells at the animal pole. Fig. 204. Viewed from the 

 posterior pole. 76 cells, viz., 32 ectoderm, !< endoderm, 18 mesoderm, 8 chorda, 8 neural 

 plate; when the divisions indicated in the ectoderm cells are completed there will be 64 

 ectoderm cells and 108 cells in all. 



Fig. 205. Gastrula corresponding in stage to fig. 140 of the Cynthia egg; 178 cells, vis., 108 ectoderm 

 and neural plate (ventral), 20 endoderm, 18 mesenchyme, 12 muscle, 8 chorda, 12 neural 

 plate (dorsal). 



Fig. 206 and 207. Older gastrula with blastopore much restricted by the growth of the anterior lip 

 Fig. 206. View of superficial ectoderm and mesoderm. Fig. 2(17. Deeper view of 

 same embryo showing endoderm and mesoderm. 



