DEVELOPMENT OF THE AMERICAN ALLIGATOR— REESE 35 



less area than the preceding. The ectoderm shows no unusual 

 features ; it is of uniform thickness except where it becomes con- 

 tinuous with the entoderm around the mandibular folds (md) ; 

 there it is somewhat thickened. The most striking feature of the 

 section is the presence of two large auditory vesicles (o). The sec- 

 tion being not quite at right angles to this part of the embryo, the 

 vesicles are not cut in exactly the same plane ; the one on the left is 

 cut through its opening to the exterior, while the one on the right 

 appears as a completely enclosed cavity. In a section a short dis- 

 tance posterior to this one the appearance of the vesicles would be 

 the reverse of what it is here. As may be seen in the figure, the 

 vesicles are large, thick-walled cavities lying close to the lateral 

 walls of the hindbrain. The hindbrain itself has the usual trian- 

 gular cross-section, with thick lateral walls and a thin, wrinkled 

 dorsal wall. Close to the ventral side of the hindbrain lies the noto- 

 chord (nt), on each side of which, in the angle between the brain 

 and the auditory vesicles, is a small blood-vessel (bv). Ventral to 

 these structures and close to the dorsal wall of the pharynx (ph) 

 are the two large dorsal aortse (ao). The ventral side of the section 

 passes through the open anterior end of the pharynx (ph). On the 

 left is seen the widely open hyomandibular cleft (#'), between the 

 main body of the section and the mandibular arch (md). On the 

 right side the plane of the section was such that the hyomandibular 

 cleft was not cut through its external opening. In each mandibular 

 fold a large aortic arch (ar) is seen, and also a slight condensation 

 of mesoblast, 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 determined. The 

 spinal cord (sc) and notochord (nt) 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 (cut), 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., 



