48o CHORD AT A. 



embryonic epiblast^ the amnion and the serosa. In perhaps 

 the simplest (pig, rabbit) the disc sinks in towards the under- 

 lying hypoblast and the walls coming up on either side as 

 folds meet above and fuse. The disc then becomes the 

 embryonic epiblast, the inner walls of the folds become the 

 amnion and the outer form part of the serosa. Thus the 

 central disc is solely the embryonic epiblast and the rest of 

 the epiblast or extra-embryonic epiblast forms the serosa and 

 the amnion. In a similar manner the hypoblast-disc forms 

 the embryonic hypoblast and the remainder, the extra- 

 embryonic hypoblast^ forms the yolk-sac only. 



The embryo is formed from the epiblastic and hypo- 

 blastic discs, the former bending over and surrounding the 

 latter. The hypoblast also bends up to form the alimentary 

 canal, and both epiblast and hypoblast become nipped off 

 from the amnion and yolk-sac, respectively, by folds. The 

 mesoblast arises between these layers around a primitive 

 streak at the blastoporic pole, and the organs arise from the 

 three layers very much as in the chick. We may here merely 

 recall the fact that the epiblast gives rise to epidermis, 

 nervous system and stomodaeum ; the hypoblast to the epi- 

 thelium of the alimentary canal and appended glands and 

 organs ; and the mesoblast to the muscles, skeleton, con- 

 nective tissue and blood-vascular system. 



The mesoblast later grows outwards from the embryo to 

 cover the embryonic membranes, creeping out as a sheet 

 over the surface of the amnion and yolk-sac and eventually 

 reaches the serosa. The outer layer of mesoblast now 

 invests the amnion and the upper part of the serosa, 

 whilst the inner layer covers the upper half of the yolk- 

 sac. At the edge the two layers meet and extend as an 

 unsplit sheet of mesoblast still further down between the 

 yolk-sac and the serosa. Further down still the serosa and 

 yolk-sac are still closely apposed and there is no mesoblast. 

 Hence the blastocyst wall is now formed (i) at its upper 

 half by a wall of epiblast and a single layer of mesoblast, 

 the completed serosa ; whilst (2) below the equator there is 

 a broad zone consisting of the epiblast of serosa, a double 

 layer of mesoblast and a layer of hypoblast (yolk-sac), all 

 in close contact; and (3) the lower pole or cap consisting 

 of epiblast (serosa) and hypoblast (yolk-sac). This is an 



