COMPARISON OF THE GERMINAL LAYERS. 249 



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 Teleostei. 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 Ascidians, the primitive row of 

 epiblast cells, when single, necessarily 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 primitive 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 equivalent 

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

 question in which of them the epiblast retains its primitive 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 decidedly 

 in favour of regarding the condition of the epiblast in the larger 

 group as primitive, and its condition in the smaller group as 

 secondary, and due to the throwing back of the differentiation of 

 the epiblast to a very early period of development. 



In favour of this view may be urged (1) the fact that the simple 

 condition is retained in Amphioxus through life. (2) The correlation 

 in Amphibia, and the other forms belonging to this group, between 

 a closed auditory pit and the early division of the epiblast into two 

 strata ; there being no doubt that the auditory pit was at first per- 

 manently open, a condition of the epiblast which necessitates its 

 never having an external opening must clearly be secondary. (3) It 

 appears more likely that a particular genetic feature should be thrown 

 back in development, than that such an important feature, as a 

 distinction between two primary layers, should be absolutely lost 

 during an early period of development, and then re-appear in later 

 stages. 



