OKGAXIXATION AND GELL-LIXKAGE OF ASCIDIAN EGG. 79 



tively small ami the degree of differentiation high as compared with Amphioxus ; 

 e.g.. at the stage when invagination begins in Amphioxus there arc according to 

 Wilson about 512 cells, at a corresponding stage in Ciona 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 deirree of their differentiation. 



In a general way the same kinds of likenesses ami differences exist between 

 ascidians and amphibians in the matter of cleavage as between the former and 

 . Xmphioxus. 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. Bias tula and Gastrula. 



The firm of the blastula and gastrula is much influenced by the relative 

 amount of yolk in different cases. A 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 Amphioxus 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 



