318 The Alligator and Its Allies 
STAGE XVII 
FIGURES 20-20j (PLATES XXV., XXVI.) 
The superficial changes noted in this stage 
chiefly concern the head, which has increased 
considerably in length (Fig. 20). The curvature of 
the body is slightly more marked, and the tail is 
more tightly coiled at the end. There are still 
signs of three gill clefts. The maxillary process 
(mx) is long and narrow, while the mandibular 
arch (md) is still short and broad. The fronto- 
nasal region has greatly increased and has the 
aquiline profile noted by Clarke. The nasal 
groove has disappeared, and there remains the 
small opening (z) at the side of the fronto-nasal 
region, near the end of the still separate maxillary 
process. The umbilicus is in about the same 
condition as in the preceding stage, but the heart 
is less prominent. The outline of the manus (ma) 
is more definite, and the extremity of the posterior 
appendage is distinctly flattened out to form the 
rudimentary pes (pe). The position of the elbow- 
joint in the anterior appendage is seen at the end 
of the reference line aa. 
Typical transverse sections of this stage are 
shown in Figures 20a-7. 
Figure 20a is a section through the middle 
region of the head, cutting the hindbrain on one 
side and the forebrain on the other. The walls of 
the brain show rather more histological differen- 
