204 University of California Publications in Zoology. [VOL. 4 



from the regular mode of cleavage described in the foregoing 

 paper." By "regular mode" he means bi-lateral. He adds 

 further : "In having a perfectly definite and stereotyped manner 

 of cleavage, the Ascidian egg resembles more closely the egg of 

 Annelids, Mollusks, and the great majority of invertebrates than 

 it does Amphioxus and the vertebrates, notwithstanding that the 

 end product of cleavage shows unmistakably the now generally 

 admitted closer affinity of the tunicates with the latter group of 

 animals." 



One must conclude from this and from what has been indi- 

 cated in the comparison of D. pusillus with Amphioxus that, in 

 so far as resemblance in cleavage may be relied upon for deter- 

 mining affinities, there is a closer relation between these two 

 animals than between Ascidia (Ciona) and Amphioxus. 



Blast ula. The blastocoele appears as a segmentation cavity 

 as early as the fourth cleavage, so that it is difficult to set apart 

 the stages of cleavage, just described, from the blastula. 



I shall regard the stage just succeeding that shown in fig. 9, 

 as the early blastula stage, for it corresponds to the earliest stage 

 that Bateson found beyond the first cleavage and which he calls 

 the blastosphere. He describes it as spherical, with opaque walls, 

 though "the outline of cells composing them could be faintly 

 distinguished in a surface view. ' ' 



I did not find among my living stages of the blastula the 

 elliptical form described by him for the later stages; but in 

 preservation some became elliptical. I did, however, observe the 

 slightly flattened condition which he describes as just preceding 

 gastrulation. 



Gastrula. Estimated from the time eggs were taken from the 

 burrow when segmentation no doubt was beginning, until the first 

 evidence of gastrulation, about twenty hours elapsed (figs. 10 

 and 11). Bateson 's account of external changes of this period 

 agrees largely with my own observations. 



The flattened condition of the gastrula remains but for a short 

 time. Before the ring of cilia appears the gastrula is nearly 

 spherical. In this respect it differs from B. kowalevskii. 



The time between the beginning of gastrulation and the ap- 

 pearance of cilia is about twelve hours. The blastopore can no 



