DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 397 
aberrantes, vient confirmer la loi générale que nous sommes 
efforcé d’établier dans d’autres études, 4 savoir que le méso- 
derme provient de |’entoderme primitif” (9). 
Figs. 13 to 13 G represent sections of ova from the same 
uterus. The ova are those of a rat, and they are in a state of 
development corresponding to about the early part of the 
second half of the eighth day in the mouse. Figs. 13 to 13 F 
represent sections of an ovum in which the embryonic area 
was 266 u long. Fig. 18 Gis a section of an ovum with an 
embryonic area 270 » long, and therefore probably a little 
further developed than the ovum represented in figs. 18 to 13 F, 
although it was contained in the same uterine cavity. 
Mesial longitudinal sections, figs. 13 to 13 B, at this period 
reveal clearly the several constituent portions of the ovum. 
The proximal part of the trophoblast (7. P.), which projects into 
the uterine crypt, to the wall of which it is attached, rests by 
its flange-like margin on the edge of the yolk-sac. The distal 
portion of the trophoblast (7. D.) is enclosed with the inva- 
ginated hypoblast, and rests by its distal free margin upon 
the epiblast. 
The epiblast on the caudal side of the cylinder(C. 4. S, figs. 
13 A and 138 B, Pl. XXIIT) is slightly thicker than that on the 
cephalic side (CHS). In the distal portion of the caudal half 
of the epiblast there is a very narrow neurenteric canal (N. C., 
fig. 13.A, Pl. XXIII), which passes obliquely through the epiblast 
into the cavity of the yolk-sac. The epiblast is entirely sepa- 
rate from the hypoblast, except at the margins of the neuren- 
teric canal, where the two layers are fused. In the middle 
line immediately distal to (in front of) the neurenteric canal 
the caudal part of the epiblast is thickened, and immediately 
proximal (posterior) to the neurenteric canal the caudal 
portion of the epiblast is thinner than elsewhere (fig. 13 A, 
Pl. XXIII). At the proximal end of its caudal half (posterior 
end of embryonic area) the epiblast is thicker than in any 
other part of its extent, and in this situation it forms a distinct 
projecting knob-like process (fig. 13 B, Pl. XXIII). 
The invaginated hypoblast (HY J., figs. 138 and 13 B) consists 
