ORGANIZATION AND CELL-LINEAGE ОҒ ASCIDIAN EGG. 19 
tively small and the degree of differentiation high as compared with Amphioxus ; 
е. g., at the stage when invagination begins in Amphzoxus there are according to 
Wilson about 512 cells, at a corresponding stage in Стома there are 76 cells. It 
may be presumed that the relative constancy or variability of cleavage in these 
two classes depends upon the two features just contrasted, viz., the number of the 
cleavage cells and the degree of their differentiation. 
In а general way the same kinds of likenesses and differences exist between 
ascidians and amphibians in the matter of cleavage as between the former and 
Amphioxus. Among amphibians, however, these differences are further increased 
by the presence of a relatively large quantity of yolk. Whether the ectoderm 
comes entirely from the four upper cells of the 8-cell stage in these animals cannot 
be affirmed, but it is evidently derived in chief part from these cells. 
4. Blastula and Gastrula. 
The form of the blastula and gastrula is much influenced by the relative 
amount of yolk in different cases. А large coeloblastula, such as is present in 
Amphioxus, does not occur in the ascidians or amphibians. In the ascidians this 
is due not merely, nor largely, to the amount of the yolk but rather to the shape of 
the cells which are always elongated either at one pole or the other so as to nearly 
fill the blastocoel ; the latter is small at all stages and the embryo and larva very 
compact. In the amphibians the relatively small size of the blastocoel is due 
not only to the quantity of yolk, but also to the many-layered character of the 
blastula wall. 
In all three classes the ectoderm arises from the upper hemisphere of the blas- 
tula, the endoderm and mesoderm from the lower hemisphere, but the precise rela- 
tion of these germ layers to the third cleavage plane is not known in the cases of 
Amphioxus and the Amphibia. 
Most investigators affirm that the gastrula invagination in Андан is at 
first radially symmetrical, and only in the later stages does it become unsymmet- 
rical. Samassa (1898), on the contrary, finds that the gastrula is bilateral from the 
beginning and concludes that this bilaterality is the direct outcome of the bilater- 
ality of the cleavage stages. In both ascidians and amphibians it is bilateral from 
the first, the invagination appearing near the anterior border of the dorsal face and 
then extending so as to include most of the dorsal area. 
5. Closure of Blastopore. 
In ascidians the closure of the blastopore results largely from the progressive 
posterior growth of the anterior (dorsal) lip, while the posterior (ventral) lip 
remains relatively fixed in position. Owing to the peculiar differentiation of the 
cells of the blastopore lip they can be individually followed through a large part of 
this process; the number of cell rows between the posterior lip and the animal 
pole and between the anterior lip and that pole can also be determined with accu- 
racy during the earlier stages of the closure; from both of these facts it is certain 
