II.] 



THE BODY-FOLDS. 31 



forms the still wide splanchnic stalk. At the end of the stalk, which is as yet 

 short, it bends outwards again and spreads over the top of the yolk. The 

 somatopleure folded in less than the splanchnopleure to form the wider somatic 

 stalk, sooner bends round and runs outwards again. At a little distance from 

 both the head and the tail it is raised up into a fold, af, af, that in front of the 

 head being the highest. These are the amniotic folds. Descending from either 

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

 into an un cleft 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 amuiotic told, 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 (7), 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 aliautois to extend from 

 its present position into the space between the two walls 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 Jimbs, 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 black 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 somato- 

 pleure 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 at first to form the body-walls 

 (which are here made too thick), soon bends outwards again, and almost im- 

 mediately is raised up into the lateral folds of tbe amnion af. The conti- 

 nuity of the pleuroperitoneal cavity within the body with the interior of the 

 amniotic fold outside the body is evident; both cavities are dotted. It will 

 of course be understood that this is a purely diagrammatic representation, 

 the various cavities, &c., being exaggerated in order to shew their relations more 

 clearly. 



G, 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) 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. 



