264 The Alligator and Its Allies 
the wide, dorsal opening (b/p) of the blastopore or 
neurenteric canal into the medullary groove (mg) is 
shown. The blastopore or neurenteric canal, then, 
is still at this stage a passage that leads entirely 
through the embryo, the medullary canal being in 
this region unenclosed above. Ventrally it is seen 
as a narrow opening through the entoderm; it 
then passes upward and backward, behind the end 
of the notochord, as a small but very distinct canal, 
which may be traced through about ten sections. 
The enclosed portion of the canal lies, as has been 
stated (Figure 9, blp), inthe center of the mass of 
cells that is fused with or is a part of the floor of 
the medullary groove. 
The above-described neurenteric canal is essen- 
tially likethat described by Balfour in the Lacertilia. 
He does not say, however, and it is not possible 
to tell from his figures, whether there is a long, 
gradually diminishing groove posterior to the 
dorsal opening of the canal, as in the alligator. 
He says that the medullary folds fuse poster- 
iorly until the medullary canal is enclosed over 
the opening of the neurenteric canal; also that 
“‘the neurenteric canal persists but a very short 
time after the complete closure of the medullary 
canal.”’ 
In Figure 9m, for about thirty sections (one 
tenth the entire length of the embryo), behind the 
section represented in the last figure, there is a very 
gradual change in the embryo, converting the deep 
