

PRELIMINARY ACCOUNT. 



[chap. 



It will be seen that the amnion a now forms in every direction the termina- 

 tion of the somatopleure ; the peripheral portions of the somatopleure, the 

 united outer or descending limbs or Avails of the folds af in 0, D, F, G having 

 been cut adrift, and now forming an independent continuous membrane, the 

 chorion, 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 

 lung been closed in), and then spreading outwards as usual 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 chorion. 



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



N 



CI 



tut 



--PP 



M 



In L the splanchnopleure has completely invested the yolk-sac, but at the 

 lower pole of the yolk is still continuous with that peripheral remnant of the 

 .somatopleure now called the chorion. In other words, cleavage of the meso- 

 blast has been carried all round the yolk (ys) except at the very lower pole. 



In M the cleavage has been carried through the pole itself; the peripheral 

 portion of the splanchnopleure forms a complete investment of the yolk quite 

 unconnected with the peripheral portion of the somatopleure. which now exists 

 as a continuous membrane lining the interior of the shell. The yolk sac (ys) is 

 therefore quite loose in the pleuroperitoneal cavity, being connected only with 

 the alimentary canal (a') by a solid pedicle. 



