l«08] Davis.— Life-history of Dolichoglossus. 203 



observations as above recorded were made upon few individuals 

 they are sufficient to warrant pointing out the great similarity 

 between the early cleavage of this animal and Amphioxus. A 

 reexamination of plenty of material and a careful study of the 

 cell-lineago of I). pusiJhis would no doubt show a greater like- 

 ness. The points of similarity are : 



(rt) Bi-lateral form of cleavage, as a rule. 



(&) Occasional variation from this into spiral, and perhaps 

 into radial and mixed cleavage. 



(c) Frequent occurrence of a cleavage pore. 



(d) Gradual obliteration, in later stages, of bi-lateral sym- 

 metry. 



The points of difference are: 



(a) Absence of radial cleavage in D. j^t'sillus. But since 

 there was one instance of mixed cleavage, it is not unlikely that 

 in a large number of individuals in early cleavage some of this 

 form might be found. 



(6) Less difference in the size of the blastomeres of the two 

 poles of the third cleavage in D. pusillus than in Amphioxus. 

 This point, however, is unessential since the yolk is uniformly 

 distributed in the eggs of both animals, and the stages subsequent 

 to the third cleavage are alike as far as can be shown with the 

 material at hand. 



Comparison of D. 2)nsillHS and Amphioxus with Ascidia (Ciona 

 intestinalis) in their early cleavage. Castle and Conklin were 

 able to recognize that the unsegmented ovum of Ascidia is made 

 up of two unlike hemispheres, one richer in yolk and the other 

 richer in protoplasm. Castle '96 concludes: "The form and 

 rate of cleavage are therefore manifestly predetermined by the 

 internal constitution of the ovum." In the Enteropneust egg 

 (D. pusillus) and in the egg of Amphioxus no such distribution 

 of yolk and protoplasm occurs. 



With reference to symmetry of cleavage Castle '96 (p. 233) 

 says: "Wilson '94 observed that the cleavage of Amphioxus 

 showed all gradations between a perfectly radial, a bi-lateral, and 

 even a spiral form ; and raised a query whether the same might 

 not be true for Ascidians. In Ciona at least this does not seem 

 to be the case. I have never observed an instance of deviation 



