A VES. 



157 



now called the serous membrane. In other words, cleavage of the mesoblast 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-sack [ys) is therefore quite loose in the 

 pleuroperitoneal cavity, being connected only with the alimentary canal (a') by a 

 solid pedicle. 



Lastly, in N the yolk-sack {ys) is shewn being withdrawn into the cavity of the 

 body of the embryo. The allantois is as before, for the sake of simplicity, omitted ; 

 its pedicle would of course lie by the side of ys in the somatic stalk marked by the 

 usual dotted shading. 



It may be repeated that the above are diagrams, the various spaces being shewn 

 distended, whereas in many of them in the actual egg the walls have collapsed, and 

 are in near juxtaposition. 



drawn backwards over the developing head of the embryo. The fold 

 thus covering the head is in due time accompanied by similar folds of 

 somatopleure, starting at some little distance behind the tail, and at 

 some little distance from the side (fig. 121, C, D, E, F and 116 am). 

 In this way the embryo becomes surrounded by a series of folds of 

 thin somatopleure, which form a continuous wall all round it. All 

 are drawn gradually over the body of the embryo, and at last meet 

 and completely coalesce (fig. 121, H, I and 117 Am), all traces of 

 their junction being removed. Beneath these united folds there is 

 therefore a cavity, within which the embryo lies (fig. 121, H ae). 

 This cavity is the cavity of the amnion. 



Each fold is necessarily formed of two limbs, both limbs consist- 

 ing of epiblast and a very thin layer of mesoblast ; but in one 

 limb the epiblast looks towards the embryo, while in the other it 



