DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 405 
The inner and middle layers have, therefore, grown simul- 
taneously with the increase in size of the ovum. The changes 
in the epiblast, however, do not indicate any very great 
activity in that layer. In sections of the distal end of the 
cylinder, where the extension of the cavity has not been great, 
the epiblast is a uniformly thick ring (fig. 16 B) continuous 
with the mesoblast in the primitive streak on the caudal side 
(CA S.) of the section, and abutting directly against the thick- 
ened hypoblast in the middle line on the cephalic side (CE 8S.) ; 
but towards the proximal end, where the cylinder is most 
dilated, the epibiast gradually becomes thinner, first on the 
lateral aspects (figs. 16 C and 16 D), and then on the cephalic 
side of the cylinder (fig. 16 #), where it becomes reduced first 
to a thin layer of nucleated protoplasm, and in ova at a little 
later period to a thin layer of flattened squames. 
The mesoblast is continuous with the epiblast and hypo- 
blast in the primitive streak from its anterior extremity 
(fig. 16 A) backwards for a distance of 23°9 m, and for the 
same distance it is still adherent to the hypoblast at the sides 
of the streak (fig. 16 B). Further backward in the embryonic 
area to the end of the primitive streak, a distance of 311 y, 
the mesoblast is separated from the hypoblast, but is con- 
tinuous with the epiblast (figs. 16 C to 16H, Pl. XXV, and 
figs. 16 G and H, Pl. XX VI). 
In front of the primitive streak the mesoblastic plates end 
at the side of a rod-like thickening of the hypoblast (figs. 16 4 
to 16 D). 
In the distal portion of the cylinder the mesoblast extends 
as a thick continuous layer from the primitive streak on the 
caudal side to the margins of the hypoblastic ridge on the 
cephalic side (fig. 16.B); but 120 ym from the end of the 
embryonic area, where the cylinder is expanded and the 
lateral epiblast much thinned, the mesoblast presents indica- 
tions of separation into two portions on each side (fig. 16 C, 
right side), and at 55 » from the end of the embryonic area the 
separation is completed on the right side (fig. 16D). One 
portion on each side belongs to the cephalic end of the 
VOL. XXXIII, PART I1l.—NEW SER. EE 
