Xx 1 W 
DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 3893 
trophoblast of the mouse becomes more extensive, and in the 
rat the cavities of the epiblast and trophoblast meet and fuse ; 
thus in both animals the epiblast and distal trophoblast assume 
the form of hollow cylinders closed at one end (fig. 18). 
The epiblastic cylinder is closed at its distal end, the tro- 
phoblastic at its proximal, and the open ends of the two 
cylinders are in close apposition, but not indistinguishably 
fused, for the characters of each portion of the ectoderm, after 
treatment with carmine, are still quite distinctive, the proto- 
plasm of the trophoblast being tinged much more faintly than 
that of the epiblast. 
For a time the united cavities of the epiblast and tropho- 
blast increase in size, together with the general growth of the 
ovum, and this increase continues until in the latter part of 
the eighth day the mesoblast appears round the margin of the 
epiblast where it is in apposition with the trophoblast (fig. 14, 
Pl. SXH),~ 
The growth of the mesoblast causes a constriction of the 
opposed margins of the epiblast and trophoblast; in other 
words, it gives rise to the amnion folds (Am.), which are small 
on account of the contracted margins of the embryonic field. 
It is at this period, the latter part of the eighth day, that it 
is possible to recognise for the first time the exact position of 
the embryonic area and its relations to the uterine walls. 
During the preceding stages the epiblast has grown in a 
narrow cylindrical uterine crypt, and has assumed a corre- 
sponding cylindrical form. The long axis cylinder lies at 
right angles to the long axis of the uterine canal, and it may 
for convenience be considered to possess four surfaces—two 
directed towards the sides of the uterine cavity, and two 
towards its extremities. At the proximal end of one of the 
former surfaces a nodular mass of mesoblast appears (fig. 14, 
Pl. XXIV), indicating the posterior extremity of the primitive 
streak, and therefore the posterior end of the embryonic area. 
From the posterior nodular mass of mesoblast two wing-like 
plates rapidly extend round the sides of the cylinder and meet 
on the opposite surface, just where the epiblast and tropho- 
