The Development of the Alligator 287 



seen, and also a slight condensation of meso- 

 blast, the latter probably being the forerunner of 

 cartilage. 



Figure 13c passes through the anterior part of 

 the heart about seventy-five sections posterior to 

 Figure 13&. The embryo in this region is narrow 

 but deep (dorso-ventrally) , the depth being largely 

 due to the size of the heart. The ectoderm (ec) 

 is considerably thickened on each side of the 

 pharynx (ph) ; this thickened area may be traced 

 for some distance both anteriorly and posteriorly 

 from this point ; its significance could not be deter- 

 mined. The spinal cord (sc) and notochord (jtt) 

 need no special description; the former is smaller 

 and the latter larger than in the more anterior 

 sections. The two large blood-vessels (ac) near 

 the spinal cord and notochord are probably the 

 anterior cardinal veins. The aortae are cut by the 

 plane of this section just anterior to their point of 

 fusion into a single vessel. A few blood corpuscles 

 are seen in the right aorta. The enteron (enl) , cut 

 posterior to the region of the gill clefts, is a large 

 elliptical cavity, with its long axis in a transverse 

 position. Its entodermal wall is comparatively 

 thin and smooth, with the cell nuclei arranged 

 chiefly on the outer side, i. e., away from the 

 cavity of the enteron. The body cavity (be) is 

 here still unenclosed, and its walls, the somatic 

 stalk, are cut off close to the body of the embryo. 

 The heart (ht), the most conspicuous feature of 



