The Development of the Alligator 269 
medullary canal is closed both above (mc) and 
below (mc’). The amnion (a) has about the same 
appearance as in the more anterior section, but 
there is here a considerable space, filled with meso- 
blast (mes), between the nervous (7) and epidermal 
(ep) layers of ectoderm. 
Figure l1c is twenty sections, about one tenth 
the length of the embryo, posterior to the one 
last described. The large mass of overhanging 
yolk (y) is still present, as is also the amnion (a), 
though the latter no longer passes entirely around 
the embryo; only the true amnion could be made 
out. The thickened walls of the medullary canal 
have reduced that cavity to a narrow, Y-shaped 
slit (mc). The notochord (ut) is very slender 
in this region, compared to its diameter farther 
toward the posterior end. The enteron (ent) is a 
large cavity, whose wall is made up of loosely 
arranged cells except around a median, ventral 
depression where the cells are more compact. 
This depression may be traced through ten or 
fifteen sections and may represent the beginning of 
the thyroid gland, though this point was not 
worked out with certainty. Surrounding the 
notochord and enteron is a loose mass of typical, 
stellate mesoblast cells (mes), which are cleft on 
either side to form the anterior limit of the body 
cavity (bc). Between the body cavity below 
and the enteron above, on each side, is a small 
blood-vessel (bv) which when followed caudad 
