188 FOETAL MEMBRANES. 



it speedily joins the splanchnopleure again, and the two, once more united into an 

 uncleft membrane, extend some way downwards over the yolk, the limit or outer 

 margin of the opaque area not being shewn. All the space between the somatopleure 

 and the splanchnopleure is shaded with dots, pp. Close to the body this space may 

 be called the pleuroperitoneal cavity; but outside the body it runs up into either 

 amniotic fold, and also extends some little way over the yolk. 



D represents the tail end at about the same stage on a more enlarged scale, in 

 order to illustrate the position of the allantois al (which was for the sake of simplicity 

 omitted in C), shewn as a bud from the splanchnopleure, stretching downwards into 

 the pleuroperitoneal cavity pp. The dotted area representing as before the whole 

 space between the splanchnopleure and the somatopleure, it is evident that a way is 

 open for the allantois to extend from its present position into the space between the 

 two limbs of the amniotic fold af. 



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 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. 



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 ali- 

 mentary canal a! even at the stalk (elsewhere the canal has of course long been closed 



