Fig. 106.—Diagram to illustrate the formations of the Em­bry­on­ic Mem­branes. A, am­nion; S, serous en­vel­ope; B, blasto­derm.

Fig. 107.—Transverse sec­tion through young Em­bryo of B. ger­man­ica. E, epi­blast; M, meso­blast; Y, yolk-cells.

This curvature of the embryo is closely connected with the formation of the embryonic membranes. On either side of the ventral plate a fold of the blastoderm arises, and these folds grow towards each other beneath the chorion. Ultimately they meet along the middle line of the ventral plate (fig. 106), and thus form a double investment, the outer layer being the serous envelope, the inner the amnion. Between the two the yolk passes in, as has been explained above (fig. 107).

At the same time that the embryonic membranes are forming, the embryonic layers make their appearance. The ventral plate, which was originally one-layered, forms the epiblast or outer layer of the embryo, and from this are subsequently derived the middle layer (mesoblast) and the deep layer (hypoblast).