The Development of the Alligator 301 



anterior to the tail. Four gill clefts (g'~ 4 ) are 

 now present; the most posterior one is more faint 

 than is represented in the figure, and it could not 

 be definitely determined from a surface view 

 whether or not it opened to the exterior. The 

 mandibular fold (md) is now fairly well outlined, 

 but there is as yet no sign of the maxillary process. 



Figure i6a is the most anterior of a series of 

 transverse sections made of an embryo of the 

 approximate age of the surface view just described ; 

 it passes through the tip of the forebrain (fb) and 

 shows the nasal pit (;/) of the right side. The 

 great thickening of ectoderm in the region of the 

 nasal invagination is represented by a solid line. 

 Owing to the obliquity of the section, the left nasal 

 pit was not cut. The mesoblast is quite dense and 

 contains two or three small blood-vessels near the 

 roof of the brain. The plane of this section, owing 

 to the cranial and body flexure, cut the embryo 

 also in the region of the pharynx; this part of the 

 section was, as a matter of convenience, omitted 

 from the drawing. 



Figure i6b is in reality more anterior in position, 

 considering the entire embryo, than the preceding; 

 but the region of the embryo represented is more 

 posterior, so that it is described at this point. 

 The greatly elongated outline of the brain is due 

 to its being cut through the region of flexure, so 

 that the forebrain (fb}, or, perhaps, midbrain, is 

 shown at one end, and the hindbrain (Jib) at the 



