3o6 The Alligator and Its Allies 



the mesentery (ms) , the section is quite deep dorso- 

 vent rally. The continuation of the amnion {a) 

 with the somatopleure is of course here evident. 



The most striking feature of the section is the 

 marked projection of the Wolffian ridges, though 

 no local enlargements of these ridges indicate the 

 rudiments of the limbs. A large mass of Wolffian 

 tubules (wt) is seen projecting into the upper part 

 of the body cavity on each side; close to each of 

 these masses is the posterior cardinal vein (pc), and 

 between them is the large aorta (ao). The other 

 structures are about as in the preceding section. 



Figure i6g represents a sagittal section of the 

 anterior half of the body of an embryo of this or 

 possibly a slightly younger stage of development. 

 The three regions of the brain are clearly indi- 

 cated, as well as the cavity of the spinal cord (sc). 

 The roof of the hindbrain has been made too thick 

 in the figure; it should be represented by a mere 

 line. A little mesoblast is to be seen at places 

 between the roof of the brain and the superficial 

 ectoderm. A slight invagination of the epithelium 

 (p) , between the floor of the brain and the anterior 

 end of the notochord, probably represents the 

 beginning of the hypophysis. No indication of the 

 paraphysis is yet to be seen. Extending from 

 the region of the hypophysis to the posterior 

 end of the section is the notochord {nt) ; it is much 

 vacuolated and gradually increases in thickness 

 toward the posterior, though its outline is quite 



