+ 
622 The American Naturalist. [July, 
portion of the area pellucida immediately around the primitive 
groove appears slightly darker than the rest. The anterior 
portion of the pellucida is further distinguished by the ante- 
rior crescent, ct, the “vordere Aussenfalte" of His, 25, and 
other German writers. The anterior crescent is a temporary ap- 
pearance due, according to Duval, to a series of folds of the ento- 
derm, which forms a curving row of shallow pockets, that, shining 
through, mark out the crescent. The crescent disappears a little 
later, and there arises, nearly if not quite in its place, a new fold, 
the amniotic. The similarity of position has led to the anterior 
crescent being identified by some authors with the true amniotic 
fold. 
E 
era 
> 
Fic. 13.—Longitudina! section of ar region of the primitive streak. of s Den. s ovum 
incubated six hours; after Duval. SE, 
tails x D, with higher t magnifica tion; Ec, ectoderm; Mes, sobs eg iat, 'ento- 
dem: Z, blastopore ; & w, germinal wall (keim wall); Ach, uliMénterie cà vity. 
Longitudinal and transverse sections are very instructive. We 
begin with the examination of a longitudinal section of a some- 
what younger stage, in which the blastopore is open. Later the 
ectoderm closes behind the primitive streak, as already stated, 
and spreads backward over the yolk. The section shows that 
the yolk is not divided into cells, although nuclei are scattered 
through it; the nuclei are represented as black dots in A and C. 
The cavity of the archenteron, Ach, is enlarged by the formation 
of a en pit in the yolk, while the posterior half of the cavity 
Be, 
FERAL Ue SERCO nt em 
