The Development of the Alligator 297 
cranial flexure and so shows only one region of the 
embryo, that of the hindbrain (hb), which is here of 
essentially the same structure as above described. 
On each side of the hindbrain is a large auditory 
vesicle (0); that on the left is cut through its 
center and shows the beginning of differentiation, 
its lower end being thick-walled and rounded, while 
its upper end is more pointed and has a thin, some- 
what wrinkled wall. The notochord (n#) is slightly 
larger than in the more anterior sections. Numer- 
ous blood-vessels (bv, ar) are seen in the mesoblast. 
The pharynx (ph) is here open ventrally and also 
through the gill cleft of the left side; on the right 
side the plane of the section did not pass through 
the external opening of the cleft. The mesoblast 
of the visceral folds is much more dense than 
that of the dorsal region of the section. 
Figure 15d, as is evident, is a section through 
the region of the heart, which appears as three ir- 
regular cavities (ht) with fairly thick mesoblastic 
walls (mes’) lined with endothelium (en’). The 
body wall, though consisting of but little besides 
the ectoderm (ec), completely surrounds the heart, 
and the pericardial or body cavity thus formed 
extends dorsally as a narrow space on either side of 
the foregut, giving the appearance of a rudimentary 
mesentery, though no especial development of such 
a structure would naturally be expected in this 
region of the embryo. The foregut (ent) is a 
moderately large cavity lined with a very distinct 
