246 The Alligator and Its Allies 
Figure 6h passes through the blastopore (0/p). 
The appearance of the section is almost identical 
with that of Figure 2f, already described. 
Figure 67 is five sections posteric~ to the preced- 
ing and has about the same structure as the corre- 
sponding sections in the preceding two stages, where 
this region of the embryo was injured, and hence 
not drawn. Continuous with the posterior bor- 
der of the blastopore (seen in the preceding fig- 
ure) is the deep furrow, the primitive groove (pg). 
The ectoblast (ec) bordering this groove is much 
thickened and may be called the primitive streak. 
The lower side of this primitive streak is continuous 
with the mesoblast (mes), while the entoderm (ez) 
is here entirely distinct from the mesoderm. It 
is evident that the mesoderm posterior to the 
blastopore is proliferated from the lower side of the 
ectoblast and not from the upper side of the ento- 
blast, as is the case anterior to the blastopore. 
The primitive groove gradually becomes more and 
more shallow, as it is followed toward the poste- 
rior, until it is no longer discernible; back of this 
point the primitive streak may be traced for a con- 
siderable distance, becoming thinner and thinner 
until it too disappears, and there remains only the 
slightly thickened ectoblast underlaid by the thin 
and irregular layers of mesoblast and entoblast. 
The primitive streak may be traced for a distance 
equal to about one third the distance between 
the head-fold and the blastopore. 
