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. G, which corresponds to D at a later stage, is introduced to shew the manner in which the allantois, now a considerable hollow body, whose cavity is continuous with that of the alimentary canal, becomes directed towards the amniotic fold. In H a longitudinal, and I 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 C, D, F, G having been cut adrift, and now forming an independent continuous membrane, the serous membrane, immediately underneath the vitelline membrane. In I 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_.