DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 433 
ninth day. It is the first rudiment of a portion of the embry- 
onic, and of the extra-embryonic mesoblast. It has already 
been explained that from the lateral margins of this mass two 
wing-like plates extend round the margin of the epiblast 
cylinder, until they meet on the cephalic side in front of the 
pro-amnion ; within these plates at the ninth day two small 
flattened spaces appear. An ovum at this period, which has 
been cut into forty-eight longitudinal sections, shows a ce- 
lomic space appearing (fig. 15 A, Pl. XXIV) in the seventeenth 
section, which continues to the twenty-first section. It is 
absent in the next six sections which pass through the middle 
of the embryonic area, but reappears in the twenty-eighth sec- 
tion, and again disappears at the thirty-second section. The 
two cavities soon meet on the caudal side, and, following the 
direction of the growth of the mesoblast (fig. 15 B), meet and 
fuse on the cephalic side in front of the embryonic area. 
Thus a circular space which entirely surrounds the margins of 
the embryonic area appears within the mesoblast. When first 
completed it is largest at the posterior end of the embryonic 
area (fig. 15 B). All parts of the inner wall of the ring-like 
space are gradually carried towards each other, but with 
unequal speed ; the caudal side and the portions nearest to it 
advance most rapidly, consequently all parts of the inner wall 
of the ring come together nearer to the cephalic than to the 
caudal side of the embryo, and after their disappearance the 
ring-like space is converted into a large cavity, which inter- 
venes between the amnion and the trophoblast. As _ this 
space increases towards the proximal pole of the ovum, the 
distal portion of the trophoblast recedes before it, becomes 
invaginated upon itself, and finally fuses with the proximal 
portion as the cavity of the trophoblast is obliterated (fig. 19, 
Pl. XX VII). As the distal portion of the trophoblast is forced out 
of the cavity formed by the invagination of the yolk-sac, the 
hypoblast with which it lay in contact is overspread by the 
mesoblastic wall of the colom, which in this situation is 
easily separable into two layers, an inner of flattened cells, 
aud an outer lying next the hypoblast, which is formed by a 
