112 ORGANIZATION AND CELL-LINEAGE ОЕ ASCIDIAN EGG. 
blastopore (pp. 51-55); the most laterally situated of these cells become the large 
muscle cells of the tail of the tadpole, the portion of the crescent lying nearest the 
dorsal mid-line becomes mesenchyme (pp. 61-67). Іп the overgrowth of the dorsal 
lip of the blastopore the muscle cells become separated from the mesenchyme (pp. 
69, 84-87). 
7. The chorda arises at the anterior border of the blastopore from yolk-laden 
cells which resemble endoderm (рр. 53, 61, 62, 70-72, 83, 84). 
8. The neural plate arises on the anterior side of the egg from cells of both 
hemispheres; it extends from the margin of the anterior lip of the blastopore to a 
point about one-third of the way from the equator to the animal pole (pp. 52-54, 
61-63, 66-68, 70, 72, 73, 82, 83). 
9. The nervous and muscular systems do not arise from a common base as 
claimed by Castle; there is no nerve ring around the blastopore (pp. 61, 72, 73). 
10. А comparison of the early development of ascidians with that of Amphz- 
oxus and amphibians shows that there is fundamental agreement among them in 
axial relations of egg and larva, in bilaterality of cleavage, in the method of closure 
of the blastopore and probably in the origin and position in the embryo of the 
neural plate, the chorda and the mesoderm (pp. 78—87). 
IL. CYTOLOGICAL RESULTS. 
ІК The 1 maturation spindles are peculiar; they have no centrosomes nor 
asters; they are formed entirely within the nuclear area from nuclear linin and 
chromosomes; their fibres at first radiate in all directions, and finally they form a 
barrel-shaped spindle. The chromosomes separate without any possible influence 
from centrosomes or traction on the part of spindle fibres (pp. 15, 16). 
12. In the first and second cleavages a small nuclear spindle like those pres- 
ent during maturation, lies between the two large asters, and in Czona it is quite 
distinct from them. The separation of the daughter chromosomes takes place here 
as in the maturation divisions (pp. 40, 41). 
13. The spermatozoon enters near the lower pole and rotates after entering 
the egg so that its centrosome is directed forward; the centrosome is derived from 
the middle piece of the spermatozoon and can be followed without interruption until 
it divides, at right angles to the copulation path, and gives rise to the sperm amphi- 
aster and finally to the cleavage centrosomes (рр. 22-24). А netrum is formed in 
the division of all centrosomes (p. 40). 
ПІ. ORGANIZATION оғ THE Ecc. 
14. In the ovocyte of Cynthia partita there is a peripheral layer of yellow 
protoplasm, a central mass of gray yolk, and a large clear germinal vesicle, which is 
eccentric toward the animal pole. These same parts are present in the eggs of 
other ascidians, but are differently colored (pp. 11, 12). 
15. When the wall of the germinal vesicle dissolves at the beginning of the 
maturation divisions a large amount of clear protoplasm, containing dissolved 
