112 ORGANIZATION AND CELL-LINEAGE OF 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. 61-07). In 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 (pp. 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. A comparison of the early development of ascidians with that of Amphi- 

 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. 73-87). 



II. Cytologic al Results. 



11. The 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 Ciojia 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 (pp. 22-24). A netrum is formed in 

 the division of all centrosomes (p. 40). 



III. Organization of the Egg. 



14. In the ovocyte of Cynthia parti/a 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 jiresent 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 



