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, Teleostei, 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 permanently 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 reappear in later stages.