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Fig. 9, A to N forms a series of purely diagrammatic representations introduced to facilitate the comprehension of the manner in which the body of the embryo is formed, and of the various relations of the yolk-sac, amnion and allantois.

E, also a longitudinal section, represents a stage still farther advanced. Both splanchnic and somatic stalks are much narrowed, especially the former, the cavity of the alimentary canal being now connected with the cavity of the yolk-sack by a mere canal. The folds of the amnion are spreading over the top of the embryo and nearly meet. Each fold consists of two walls or limbs, the space between which (dotted) is as before merely a part of the space between the somatopleure and splanchnopleure. Between these arched amniotic folds and the body of the embryo is a space not as yet entirely closed in.

F represents on a different scale a transverse section of E taken through the middle of the splanchnic stalk. The dark ring in the body of the embryo shews the position of the neural canal, below which is a black spot, marking the notochord. On either side of the notochord the divergence of somatopleure and splanchnopleure is obvious. The splanchnopleure, more or less thickened, is somewhat bent in towards the middle line, but the two sides do not unite, the alimentary canal being as yet open below at this spot ; after converging somewhat they diverge again and run outwards over the yolk. The somatopleure, folded in to some extent to form the body- walls, soon bends outwards again, and is almost immediately raised up into the lateral folds of the amnion af. The continuity of the pleuroperitoneal cavity within the body with the interior of the amniotic fold outside the body is evident ; both cavities are dotted. (7, which corresponds to D at a later stage, is introduced to shew the manner in which the allantois, now a distinctly hollow body, whose cavity is continuous with that of the alimentary canal, becomes directed towards the amniotic fold.

In H a longitudinal, and / a transverse section of later date, great changes have taken place. The several folds of the amnion have met and coalesced above the body of the embryo. The inner limbs of the several folds have united into a single membrane (a), which encloses a space (ae or ac) round the embryo. This membrane(a)is-the amnion proper, and the cavity within it, i.e. between it and the embryo, is the cavity of the amnion containing the liquor amnii. The allantois is omitted for the sake of simplicity. It will be seen that the amnion a now forms in every direction the termination of the somatopleure ; the peripheral portions of the somatopleure, the united outer or descending limbs of the folds af in (7, D, F, G having been cut adrift, and now forming an independent continuous membrane, the serous membrane, immediately underneath the vitelline membrane. In / the splanchnopleure is seen converging to complete the closure of the alimentary canal a' even at the stalk (elsewhere the canal has of course long been closed in), and then spreading outwards as before over the yolk. The point at which it unites with the somatopleure, marking the extreme limit of the cleavage of the mesoblast, is now much nearer the lower pole of the diminished yolk.

As a result of these several changes, a great increase in the dotted space has taken place. It is now possible to pass from the actual peritoneal cavity within the body, on the one hand round a great portion of the circumference of the yolk, and on the other hand above the amnion a, in the space between it and the serous envelope.

Into this space the allantois is seen spreading in K at al.

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Reference

Foster, M., Balfour, F. M., Sedgwick, A., & Heape, W. (1883). The Elements of Embryology. (2nd ed.). London: Macmillan and Co.

The Elements of Embryology (1883)

Volume 1 The History of the Chick: Egg structure and incubation beginning | Summary whole incubation | First day | Second day - first half | Second day - second half | Third day | Fourth day | Fifth day | Sixth day to incubation end

Volume 2 The History of the Mammalian Embryo: General Development | Embryonic Membranes and Yolk-Sac | Organs from Epiblast | Organs from Mesoblast | Alimentary Canal | Appendix