The Development of the Alligator 301 
anterior to the tail. Four gill clefts (g™) are 
now present; the most posterior one is more faint 
than is represented in the figure, and it could not 
be definitely determined from a surface view 
whether or not it opened to the exterior. The 
mandibular fold (md) is now fairly well outlined, 
but there is as yet no sign of the maxillary process. 
Figure 16a is the most anterior of a series of 
transverse sections made of an embryo of the 
approximate age of the surface view just described ; 
it passes through the tip of the forebrain (fb) and 
shows the nasal pit () of the right side. The 
great thickening of ectoderm in the region of the 
nasal invagination is represented by a solid line. 
Owing to the obliquity of the section, the left nasal 
pit was not cut. The mesoblast is quite dense and 
contains two or three small blood-vessels near the 
roof of the brain. The plane of this section, owing 
to the cranial and body flexure, cut the embryo 
also in the region of the pharynx; this part of the 
section was, as a matter of convenience, omitted 
from the drawing. 
Figure 160 is in reality more anterior in position, 
considering the entire embryo, than the preceding; 
but the region of the embryo represented is more 
posterior, so that it is described at this point. 
The greatly elongated outline of the brain is due 
to its being cut through the region of flexure, so 
that the forebrain (fb), or, perhaps, midbrain, is 
shown at one end, and the hindbrain (tb) at the 
