480 CHORDATA. 
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 sevosa. 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 muchas in the chick. We may here merely 
recall the fact that the epiblast gives rise to epidermis, 
nervous system and stomodzeum ; 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 (1) 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 
