300 THE EPIBLAST. 



differentiation of hypoblast and mesoblast in Elasmobranchii 

 without an invagination. 



In the Sauropsida, some of the mesoblast of the vascular area 

 would appear to be formed in situ out of the germinal wall, by 

 a process of cell-formation similar to that which takes place in 

 the yolk adjoining the blastoderm in Elasmobranchii and Tele- 

 ostei. The mesoblast so formed is to be compared with that 

 which arises on the ventral side of the embryo in the Frog, by a 

 direct differentiation of the yolk-cells. 



What was stated for the Elasmobranchii with reference to 

 the general fate of the mesoblast holds approximately for all the 

 other forms. 



The Epiblast. 



The epiblast in a large number of Chordata arises as a single 

 row of more or less columnar cells. Since the epidermis, into 

 which it becomes converted, is formed of two more or less 

 distinct strata in all Chordata except Amphioxus and Asci- 

 dians, the primitive row of epiblast cells, when single, neces- 

 sarily becomes divided in the course of development into two 

 layers. 



In some of the Vertebrata, viz. the Anurous Amphibia, Tele- 

 ostei, Acipenser, and Lepidosteus, the epiblast is from the first 

 formed of two distinct strata. The upper of these, formed of a 

 single row of cells, is known as the epidermic stratum, and the 

 lower, formed of several rows, as the nervous stratum. In these 

 cases the two original strata of the epiblast are equivalent to 

 those which appear at a later period in the other forms. Thus 

 Vertebrates may be divided into groups according to the primi- 

 tive condition of their epiblast, viz. a larger group with but a 

 single stratum of cells at first ; and a smaller group with two 

 strata. 



While there is no great difficulty in determining the equiva- 

 lent parts of the epidermis in these two groups, it still remains 

 an open question in which of them the epiblast retains its primi- 

 tive condition. 



Though it is not easy to bring conclusive proofs on the one 

 side or the other, the balance of argument appears to me to be 



