398 ARTHUR ROBINSON. 
of large, ill-defined, columnar cells, where it covers the distal 
portion of the trophoblast. As arule, each cell of this portion 
of the hypoblast contains a large round nucleus, but on the 
caudal side of the distal trophoblast, and immediately proximal 
to the epiblast, nuclear proliferation is proceeding in the hypo- 
blast (fig. 18 B, Pl. XXITT). 
The invaginated hypoblast (HY I, figs. 18, 18 A, and 13 B, Pl. 
XXIII) which surrounds the epiblast is much thinner than that 
which surrounds the distal trophoblast, and it is not divided 
into cell areas, except round the proximal end of the epiblastic 
cylinder (fig. 13 B), where the hypoblast which lies in contact 
with the epiblast gradually merges into and assumes the same 
characters as the hypoblast which surrounds the trophoblast. 
At the margins of the neurenteric canal this layer of the hypo- 
blast becomes continuous with the epiblast (fig. 13 A). 
The uninvaginated hypoblast (HY #., fig. 13) consists of a 
single layer of flattened cells, which are spindle-shaped in 
section. Proximally it is slightly overlapped by the flange- 
like margin of the proximal portion of the trophoblast, but in 
the remainder of its extent it lies in direct contact with the 
maternal tissues, and in many places is bathed by maternal 
blood (42). 
Transverse sections of the epiblast cylinder and the sur- 
rounding hypoblast render still more clear the peculiarities 
observable at this period. 
Fig. 13 C is the twelfth of a series of forty-seven sections of 
an epiblastic cylinder 266 yw long. It passes through the 
cylinder immediately proximal to the neurenteric canal, and it 
confirms the appearances seen in fig. 13 4 in the same position. 
On the caudal side of the cylinder the epiblast is thinnest, 
apparently on account of a depression of its internal surface. 
This depression is the anterior extremity of the primitive 
groove, which extends backwards (towards the proximal end 
of the caudal surface) for a distance of 17 mw; it is present, 
therefore, on sections 13 and 14, on the latter of which it is 
very shallow, but it is not found on section 15. 
Behind the posterior extremity of the primitive groove the 
