394 ARTHUR ROBINSON. 
blast are in contact (fig. 15, Pl. XXIV); and at the first glance 
it appears probable that the point immediately behind the 
fusion of the mesoblastic plates is the anterior end of the em- 
bryonic area. Fraser and Duval have looked upon it in this 
light. If this supposition were correct it is evident that the 
anterior and posterior extremities of the embryonic area would 
be directly opposite to each other, and that the central point 
of the distal end of the cylinder would be the central point of 
the embryonic area. This is, however, not the case, for it 
soon becomes evident that a portion of the cylinder imme- 
diately distal to the fusion of the mesoblastic plates forms for 
a time a small pro-amniotic fold (AMP, fig. 15); therefore, 
in the dorsal curvature of the embryonic area, which is one 
of the peculiar features of the inverted layers, the bend does 
not occur across, but some distance in front of the centre of 
the area, and the neurenteric canal, which is situated a short 
distance posterior to the centre of the young embryonic area, 
is well behind the bend (NC, fig. 15). Further, it will 
eventually be shown that portions of the lateral walls of the 
cylinder become split up and take part in the formation of the 
amnion ; consequently these portions also are extra-embryonic. 
In other words, within the area of the epiblast the whole of 
the embryonic and part of the extra-embryonic area are 
included. 
The only portion of the extra-embryonic area included in 
the epiblastic area is that which takes part in the formation 
of the amnion, and the constriction which separates the amnion 
from the false amnion does not pass through the epiblast as 
Selenka figures it (45, Taf. xvi, figs. 64 and 67), but between 
the epiblast and the trophoblast (see fig. 15). 
The bent long axis of the embryonic area lies in a plane 
which is at right angles with the long axis of the uterine canal. 
The bend of the embryonic axis is opposite the inferior or ven- 
tral border of the uterus ; therefore the axial portion of the 
embryonic area in front of the bend lies parallel with one sur- 
face of the uterus, and the portion behind the bend is parallel 
with the opposite surface of the uterus. One of these two 
