408 ARTHUR ROBINSON, 
line no layers are distinguishable (fig. 17, Pl. XX VI), and the 
large rounded nuclei have no definite arrangement. 
In front of the remains of the neurenteric canal the epiblast 
and hypoblast are fused for a distance of 21 p (fig. 17, Pl. 
XXVI). In this region of fusion, as in the primitive streak, the 
nuclei are large, rounded in shape, and not definitely arranged ; 
but further forward, where the two layers are separate, the 
nuclei of the hypoblastic ridge, to which attention has been 
drawn in the description of transverse sections, are generally 
large, of oval shape, and assume a somewhat palisade-like 
arrangement (fig. 17). They retain this form and arrangement 
for only a short distance, and then, when the hypoblastic ridge 
is narrowed (fig. 16 B), the nuclei are smaller and rounder ; 
still further forward they are flattened, but in the thickened 
anterior portion of the ridge (figs. 16 Cand 16 D) the nuclei 
are again large and oval. 
The nuclei of the epiblast which is immediately superjacent 
to the hypoblastic ridge are also, for the most part, oval, and 
their long axes lie at right angles to the surfaces of the layer 
(figs. 16 C, 16 D, and 17). 
During the latter part of the ninth day in the mouse, and 
at a corresponding period in the rat, the most important 
changes which take place are those in the anterior portion of 
the embryonic area. 
In a mesial longitudinal section (fig. 18 4, Pl. XXVI) an 
axial mass of mesoblast is seen between the epiblast and hypo- 
blast at the anterior end of the embryonic region. It is quite 
separate from the epiblast above it, and from the splanchnic 
mesoblast of the extra-embryonic region in front of it, though 
the latter now lies in close relation with the axial mesoblast, 
for the didermic area which previously intervened between 
them has disappeared in this embryo, but in another embryo 
from the same uterus a small portion not more than 8p in 
antero-posterior length remains (fig. 18 B).° 
At one point the inferior surface of the axial mesoblast is 
fused with the hypoblast, from which it is apparently receiving 
a further addition of nuclei. The hypoblast beneath the 
