Fig. 188. Diagrammatic longitudinal section through an embryo Lizard to shew the relations of the neurenteric canal (ne) and of the primitive streak (pr).
am. amnion; ep. epiblast; hy. hypoblast; ch. notochord; pp. body cavity; ne. neurenteric canal; pr. primitive streak.
Fig. 189. Transverse section through the embryonic region of the blastoderm of a Chick at the time of the formation of the notochord, but before the appearance of the medullary groove.
ep. epiblast; hy. hypoblast; ch. notochord; me. mesoblast; n. nuclei in the yolk of the germinal wall yk.
The division of the mesoblast into two plates along the dorsal line of the embryo, and the formation of the notochord from the axial hypoblast, are intelligible without further explanation. The appearance of part of the mesoblast before the formation of the primitive streak is a process of the same nature as the 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 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.