INHERITANCE 



3^3 



Fig. 180. Eggs of the ascidian 

 Cynthia showing differentia- 

 tion of organ forming sub- 

 stances in the cytoplasm, 

 lateral views (after Conklinj. 

 a. unsegmented, but a^ter 

 fertilization; b, in the eight- 

 cell stage. 



in their appearance and also in their destination in the tis- 

 sues, i) There is a clear protoplasm that will develop into 

 the ectoderm; 2) there is a gray, yolk-filled protoplasm that 

 will develop into endoderm, and 3) there is a yellow proto- 

 plasm that will develop into mesoderm. In figure 180 these 

 are imperfectly delimited, the yellow protoplasm being 

 diagrammatically indicated by the heavier stippling, and 



the gray by the 

 intermediate stip- 

 pling; a repre- 

 sents the egg half 

 an hour after the 

 entrance of the 

 sperm cell, but before the first division; 

 the yellow protoplasm has taken up its 

 position in a crescent across one side of the 

 egg, half of it being shown in the figure. 

 The first cleavage plane (median plane of 

 the body to be formed later) will be in the 

 plane of the paper, dividing these sub- 

 stances symmetrically; 6 is a corres- 

 ponding view in the 8-cell stage, with the 

 polar bodies still persisting at the upper 

 pole, and the yellow protoplasm occupying a part only of 

 two cells of the lower hemisphere, while most of the gray 

 protoplasm has withdrawn into the other two. The yellow 

 protoplasm will all develop later into the early muscle seg- 

 ments lying alongside the notocord. Here we have definite, 

 predetermined materials for the making of the embryo, 

 which, though differing in kind do not correspond with cell 

 boundaries and which are therefore, clearly unrelated as yet 

 to chromosome behavior. 



Synapsis. — There is another process that is believed to 

 intervene in the case of many of the higher animals and 



