3o8 The Allirator and Its Allies 



ular cleft (gO 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 (/?/) is, perhaps, 

 somewhat abnormal. The umbilical stalk (?/) is 

 comparatively narrow and, like the allantois, was 

 cut off close to the body. 1= 



Transverse sections of an embryo of this stage 

 are represented in Figures i7a-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 (fh) and posterior 

 part of the hindbrain (hb). 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- 



