3i8 The Alligator and Its Allies 



STAGE XVII 

 Figures 20-20; (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 (77) 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 (717 a) 

 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-j. 



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- 



