ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 105 



oxyebromatin, is liberated into the cell body. This clear protoplasm is eccentric 

 toward the animal pole and is distinct from the yolk and peripheral layer (pp. 

 13, 17). 



lii. Immediately after the entrance of the spermatozoon the yellow and clear 

 protoplasm How 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 covers the surface of the lower hemi- 

 sphere. This flowing of protoplasm to the point of entrance of the sperm is com- 

 parable with what takes place in many animals, though here much more extensive 

 and rapid than elsewhere (pp. 1921, 77). 



17. The withdrawal of protoplasm from the upper pole leaves the maturation 

 spindles closely surrounded by yolk. The polar bodies are thus formed at the 

 middle of a yolk-rich hemisphere, which is, however, the animal pole and not 

 the vegetal pole as was claimed by Castle (pp. 19-21, 29, 30, 36, 37, 87-90). 



18. The sperm nucleus moves from the point of entrance toward the equator 

 in a path which is apparently predetermined. Tins path lies in the plane of the 

 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 

 spermatozoon, but by the structure of the egg. All the axes of the future animal 

 are now clearly established, antero-posterior, right-left, dorso-ventral (pp. 22, 26, 

 90-93). 



19. As the sperm nucleus moves to the posterior pole the clear and the yellow 

 protoplasm move with it ; the latter collects into a yellow crescent with its middle 

 at the posterior pole and its horns extending about halfway around the egg just 

 below the equator. This position it retains throughout the whole development, 

 giving rise to the muscle and mesenchyme cells mentioned in 6 (pp. 19-21, 97, 98). 



20. After the sperm and egg nuclei have met at the posterior pole they move 

 in toward the center of the egg and the clear protoplasm goes with them ; the out- 

 place where the latter remains in contact with the surface is along the upper border 

 of the crescent. At the close of the first cleavage the nuclei and clear protoplasm 

 move into the upper hemisphere, and thereafter, throughout development, this 

 hemisphere contains most of the clear protoplasm and gives rise to the ectoderm 

 (pp. 20, 21, 42, 102). 



21. The yolk which before maturation was central in position is shifted 

 toward the animal pole when the protoplasm flows down to meet the spermatozoon ; 

 when the sperm nucleus and surrounding protoplasm move to the posterior pole 

 the yolk is moved down around the anterior side of the egg to the lower pole, and 

 when the clear protoplasm moves into the upper hemisphere the yolk is largely 

 collected in the lower hemisphere. This yolk rich area gives rise to the endoderm 

 (pp. 20', 33-35, 12, 102). 



22. At the close of the first cleavage the principal germ regions of the embryo 

 are visible in their definite positions and proportions, viz. : the muscle-mesenchyme 



15 JOURX. A. X. S. PHILA.. VOL. XIII. 



