xv] DIFFERENTIAL DIVISION 247 



four cells, each of which then undergoes a very unequal 

 division, giving rise to a small cell almost free from yolk 

 and a large yolk-laden cell. Two similar unequal divisions 

 of the large cells then follow, producing in all three quartets 

 of small cells and four large cells containing yolk (oil-drops). 

 The cap of small cells gives rise to the ectoderm; the four 

 large cells undergo no further division for some time, and 

 are the primary endoderm cells. If, however, the egg before 

 segmentation is subjected to pressure between glass plates, 

 all the cleavage furrows are vertical and the cells produced 

 lie in one plane, so that after three cleavages there are eight 

 cells all containing oil-drops. If they are now released from 

 pressure, each cell divides unequally into an upper and 

 lower portion, giving rise to eight small cells and eight large 

 oil-laden cells. The small cells then give origin to the ecto- 

 derm, and the eight large cells all become endodermal. It is 

 evident, therefore, that the nuclear division of the third 

 cleavage, which under natural circumstances would have 

 separated ectodermal from endodermal cells, under these 

 circumstances has not produced any differentiation, for all 

 the eight cells become endoderm. In other words, the nuclei 

 of four of the eight endoderm cells were naturally destined 

 to become those of ectoderm, but actually, under experi- 

 mental conditions, become part of the endoderm, from which 

 it follows that the differentiation of the germ-layers is not 

 conditioned by differential nuclear division, but by unequal 

 division of the cytoplasm. 



Many experiments have been made in order to determine 

 the function of the various substances present in different 

 parts of the egg of many animals, and from the regularity 

 with which these are often distributed to definite organs 

 or tissues, it has been concluded that they are specific 

 "organ-forming" substances. Of the Ascidian Styela, for 



