ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 105 
oxychromatin, 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). 
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 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 (рр. 19-21, 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 (рр. 19-21, 29, 50, 36, 37, 87-90). 
18. The sperm nucleus moves from the point of entrance toward the equator 
in a path which is apparently predetermined. This 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 half way 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 only 
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, 42, 102). 
22. А+ the close of the first cleavage the principal germ regions of the embryo 
are visible in their definite positions and proportions, 222. ; the muscle-mesenchyme 
15 JOURN. A. N. S. PHILA., VOL. XIII. | 
