DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 401 
In longitudinal sections of the central portion of the embry- 
onic area (fig. 14 A) it is impossible to discover any separation 
between the epiblast and hypoblast at the anterior end of the 
primitive streak. In front of the neurenteric canal both the 
epiblast and hypoblast are thickened, and in the latter layer 
the nuclei are no longer arranged in a definite single row, but 
are more or less irregularly scattered throughout the proto- 
plasm. Further forward, towards the cephalic end of the 
embryonic area, the hypoblast becomes thinner and its nuclei 
less numerous. 
Transverse sections of an ovum from the same uterus 
confirm all the most important points observable on longitu- 
dinal sections, and they give much more information concern- 
ing the formation of the mesoderm. 
Figs. 14 Bto 14 J are representations of transverse sections 
which have passed in an oblique plane through an ovum with 
an epiblast cylinder about 288 pw long. The cylinder is 
divided into sixty-one sections, and the tenth section (fig. 
14 B) passes just in front of the fusion of epiblast and hypo- 
blast in the anterior boundary of the neurenteric canal along 
the line (8) (fig. 14 A, Pl. XXIV). The epiblastic cylinder is 
surrounded on all sides by a ring of hypoblast. The epiblast is 
thickened on the caudal side (CA S.), where it projects 
towards but does not touch the hypoblast, which is thickened 
in the same region. The hypoblast, however, is not only 
thickened on the caudal side in the middle line, but also 
laterally over an area equal to about a third of the circumfer- 
ence of the section, and in the same regions its nuclei are more 
numerous than elsewhere, and they are irregularly arranged in 
two rows. 
The next section, the eleventh (fig. 14 C), passes through 
the anterior boundary of the neurenteric canal along the line 
c (fig. 14 A). On the caudal side of the section the epiblast 
and hypoblast are fused, and laterally for about half the cir- 
cumference of the section the hypoblast is thickened, and its 
nuclei are arranged in a double row. 
As the sections proceed towards the proximal end of the 
