18 The Unity of the Organism 



mar}^ of results, and then use these as the basis of a closer 

 scrutiny of the conclusions. Under the head "Organization 

 of the Egg" we find: "14. In the ovocyte of Cynthia partita 

 there is a peripheral layer of yellow protoplasm, {p. I.) a 

 central mass of gray yolk, (yk.) and a large clear germinal 

 vesicle (g. v.) which is eccentric toward the animal pole 

 (figure 43a). These same parts are present in the eggs 

 of other ascidians, but are difFerentlv colored. 



"15. When the wall of the germinal vesicle dissolves at 

 the beginning of maturation divisions a large amount of 

 clear protoplasm, containing dissolved oxychromatin, is 

 liberated into the cell body. This clear protoplasm is ec- 

 centric toward the animal pole and is distinct from the yolk 

 and peripheral layer. 



"16. Immediately after the entrance of the spermatozoon 

 the yellow and clear protoplasm flow rapidly to the lower 

 pole, where the yellow protoplasm collects around the point 

 of entrance ; the clear protoplasm lies at a deeper level. The 

 yellow protoplasm then spreads out until it coA^ers the sur- 

 face of the lower hemisphere. This flowing of protoplasm 

 to the point of entrance of the sperm is comparable with 

 what takes place in many animals, though much more ex- 

 tensive and rapid here than elsewhere (figure 48b). ^ 



*'18. The sperm nucleus moves from the point of en- 

 trance toward the equator in a path which is apparently 

 predetermined. This path lies in the plane of first cleavage 

 and the point, just below the equator, at which the sperm 

 nucleus stops in its upward movement, becomes the posterior 

 pole of the embryo. The median plane and the posterior 

 pole are probably not determined by the path of the sperma- 

 tozoon but by the structure of the egg- All the axes of the 

 future animal are now clearly established antero-posterior, 

 right-left, dorso-ventral. 



19. The yellow protoplasm "collects into a yellow cres- 

 cent with its middle at the posterior pole and its horns ex- 



