DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 3899 
epiblast is thickened in the middle line on the caudal side ; 
this thickening is specially marked from the fifteenth to the 
eighteenth sections—that is, for a distance of 23. The cha- 
racters of the thickening are shown in fig. 13 D, Pl. XXIV, 
which represents the sixteenth section. Beyond the sixteenth 
section the epiblast on the caudal side is still thicker than on the 
cephalic side, but only toa very slight extent (see figs. 18 H and 
13 F, Pl. XXIV, the twenty-fourth and thirtieth sections respec- 
tively), until the proximal end of the cylinder is reached, and 
here (fig. 13 G, Pl. XXIV) the epiblast on the caudal side 
in the middle line is much thicker than the epiblast on the 
cephalic side. 
The relations of the hypoblast to the epiblast are rendered 
still more evident by transverse than they were by the longi- 
tudinal sections. On the cephalic and caudal sides of the 
distal portion of the epiblast, the central portion of the ger- 
minal area, the hypoblast is a thin layer of nucleated proto- 
plasm, which lies in close contact with the epiblast (fig. 13 C). 
As the proximal portion of the tube is approached (figs. 13 D 
to 13 F) the hypoblast on both the cephalic and caudal sides 
becomes thicker. In other words, in the central portion of 
the embryonic area the hypoblast is a thin layer, and it is con- 
tinuous with the epiblast round the margins of a narrow neu- 
renteric canal (fig. 13.4, Pl. XXIII), which lies in the mesial 
plane of the embryonic area behind its centre. 
In the peripheral part of the embryonic area the hypoblast 
is thicker than in the central portion; but it is, up to this 
time, entirely distinct from the epiblast. Within a very short 
period, however, when the long axis of the embryonic area 
has increased to 270 p, the thickened epiblast of its posterior 
extremity fuses with the underlying hypoblast (fig. 13 G). 
This fusion appears to take place without any active participa- 
tion of the hypoblast, for at the forty-third section from the 
distal end of the caudal epiblast, where the commencement of 
the fusion is first noticeable, there is no sign of proliferation 
of the hypoblastic nuclei, and the fusion appears to be brought 
about by the gradual flowing together and admixture of the 
