DEVELOPMENT OF THE AMERICAN ALLIGATOR — REESE 4 1 



scribed. On each side of the hindbrain is a large auditory vesicle 

 (o) ; that on the left is cut through its center and shows the begin- 

 nino- of differentiation, its lower end being thick-walled and 

 rounded, while its upper end is more pointed and has a thin, some- 

 what wrinkled' wall. The notochord (nt) is slightly larger than in 

 the more anterior sections'. Numerous blood-vessels (bv, ar) are 

 seen in the mesoblast. The pharynx (pli) is here open ventrally 

 and also through the gill cleft of the left side ; on the right side the 

 plane of the section did not pass through the external opening of the 

 cleft. The mesoblast of the visceral folds is much more dense than 

 that of the dorsal region of the section. 



Figure I5(/, as is evident, is a section through the region of the 

 heart, which appears as three irregular cavities (ht) with fairly 

 thick mesoblastic walls (mes') lined with endothelium (en'). The 

 body wall, though consisting of but little besides the ectoderm (ec), 

 completely surrounds the heart, and the pericardial or body cavity 

 thus formed extends dorsally as a narrow space on either side of 

 the foregut, giving the appearance of a rudimentary mesentery, 

 though no especial development of such a structure would naturally 

 be expected in this region of the embryo. The foregut (ent) is a 

 moderately large cavity lined with a very distinct entoderm of even 

 thickness. Dorsal to the foregut are three large blood-vessels, a 

 median, and now single, dorsal aorta (ao), and a pair of cardinal 

 veins (cv). The notochord (nt) is small and is flattened against 

 the ventral side of the spinal cord (sc), which latter structure needs 

 no special mention. The muscle plates (mp) are considerably 

 elongated, so that they now extend ventrally to a point slightly 

 below the upper angles of the body cavity. 



Figure 15^ is through the middle region of the embryo, and, 

 owing to the curvature of the body, is not an exact dorso-ventral 

 section; this accounts, in part at least, for the unusual diameter in 

 a dorso-ventral direction of the aorta (ao) , which is very large in 

 proportion to the other structures. The posterior cardinal vein is 

 shown on the left, but not on the right. The relative sizes of the 

 spinal cord (sc) and notochord (nt) are very different from what 

 was seen in the preceding figure. In this section the spinal cord 

 is considerably smaller than in the preceding, while the notochord 

 is very much larger; in fact the notochord here seems abnormally 

 large when compared to corresponding sections of other series. It 

 is true, however, that while the spinal cord has been diminishing 

 in diameter the notochord has been increasing. The spinal cord, 

 notochord, and dorsal aorta are all so large that they are flattened 

 against each other, the pushing in of the ventral side of the spinal 



