273 



other a circle of three nuclei surrounding a fourth. This last is the 

 ancestor of the endoderm cells. The fourth division produces sixteen 

 nuclei, two of which belong to the endoderm. The circle of three 

 which surround the primitive endoderm nucleus have divided to 

 form a circle of six nviclei, and to this circle a seventh nucleus is ad- 

 ded by a longitudinal division of one of the four nuclei of the other 

 extremity of the egg. The endodermal pole of the egg consequently 

 presents two endoderm nuclei surrounded by a circle of seven nuclei ; 

 in the rearrangement of the nuclei which takes place before the next 

 division the number of nuclei in the circle is reduced to six by the 

 migration toward the ectodermal pole of one of those formed by a divi- 

 sion of a nucleus originally belonging to the endodermal pole. The six 

 nuclei thus arranged are the parent nuclei of the mesoderm. 



Up to this stage there has been no division planes visible at the 

 surface of the egg, but, when it is completed, the nuclei with their 



Fig. 1. 



Fig. 2. 



surrounding protoplasm have reached the surface, and the next divi- 

 sion, by which thirty-two cells are formed, shows itself by a division 

 of the yolk as well as by a division of the nuclei. The cell-divisions 

 are superficial however, the great mass of central yolk remaining un- 

 divided. A remarkable differentiation of the cells also manifests itself 

 at this stage ; the endodermal cells are increased to four , and show 

 their nuclei only or rather their protoplasm is not concentrated around 

 their nuclei, but is disseminated throughout the yolk. These are sur- 

 rounded by a circle of twelve mesoderm cells, Avhose protoplasm is 

 strongly concentrated around the nuclei, while the rest of the egg is 

 occupied by sixteen scattered cells in which the degree of concentration 

 of the protoplasm around the nuclei is intermediate between that 

 occurring in the mesoderm and endoderm cells. Fig. 2 will afford a 



