308 The Alligator and Its Allies 
ular cleft (g’) still persists as a small crescentic slit, 
while the next three clefts are now represented 
merely by superficial grooves separated by distinct 
ridges, the visceral folds. No indication of a fifth 
cleft is seen. The maxillary process (mx) grows 
ventralward under the forebrain and is already 
longer than the mandibular arch (md). 
The chief advance in development over the 
preceding stage, besides the formation of the 
maxillary process, is in the appearance of the 
appendages (aa and pa); they have the char- 
acteristic shape of the rudimentary vertebrate 
appendage, though the anterior pair seem to point 
in an unusual direction at this stage and to be 
slightly more developed than the posterior. The 
curious, anteriorly directed heart (ht) is, perhaps, 
somewhat abnormal. The umbilical stalk (u) is 
comparatively narrow and, like the allantois, was 
cut off close to the body. 
Transverse sections of an embryo of this stage 
are represented in Figures 17a—-g, drawn under a 
lower magnification than were any of the preceding 
figures. 
Figure 17a is in the region of the pharynx, and 
passes through the forebrain (fb) and posterior 
part of the hindbrain (ib). In the thick walls of 
both of these structures histological differentiation 
has begun, so that even under low power an inner 
granular and an. outer clear zone may be distin- 
guished. Under greater magnification the pres- 
