DEVELOPMENT OF THE AMERICAN ALLIGATOR — REESE 31 



of the embryo was so thin and indistinct that it could scarcely be 

 distinguished from the mesoderm of. that region. The amnion (a) 

 is still a continuous envelope entirely surrounding the embryo. 



Figure \2.d, about twenty sections posterior to figure 12c, is in 

 the posterior hear*t region. The spinal cord (sc), as might be ex- 

 pected, is smaller than in the more anterior region, but is otherwise 

 not markedly different from what was there seen. The notochord 

 (lit) also has the. same appearance as before. The enteron (ent) 

 shows of course in this region no gill clefts ; it is a small, irregular 

 cavity with thicker walls than in the figure just described. The 

 ventro-lateral depression is entirely distinct from the depression that 

 was called the thyroid rudiment in the preceding figure. Dorsal to the 

 enteron are the two dorsal aortse (ao), now smaller and more ventral 

 to the notochord than in the preceding figure. Ventral to the enteron 

 is the large heart (lit), projecting below the body cavity, which is no 

 longer enclosed. The mesodermic wall (mes f ) of the heart is still 

 comparatively thin and is separated by a considerable space from 

 the membranous endocardium (en'). The extent and shape of the 

 heart are shown in the surface view of this stage. On the right side 

 of the section the body cavity extends to a point nearly opposite the 

 middle of the spinal cord, considerably dorsal to the notochord, 

 while on the left side the dorsal limit of the body cavity is scarcely 

 level with the lower side of the notochord. Between the dorsal end 

 of the body cavity and the side of the spinal cord, on the left, is a 

 dense mass of mesoblast (s), one of the mesoblastic somites. A few 

 sections either anterior or posterior to the one under discussion will 

 show the condition of the two sides reversed — that is, the body 

 cavity will extend to the greater distance on the left and will be 

 interrupted by a mesoblastic somite on the right. It is evident, 

 then, that the upper angle of the body cavity is extended dorsally 

 as a series of narrow pouches between the somites. The mesoblast 

 that lines the body cavity, the splanchnopleure (sin), and somat- 

 opleure (so) is somewhat denser than the general mass of meso- 

 blast, so that these layers are quite distinct, the former (sm) extend- 

 ing around the enteron (ent) and heart (lit), and the latter (so) 

 being carried dorsal ward as the mesoblastic part of the amnion (a). 

 The amnion may be traced through about 130 of the 200 sections 

 into which this embryo was cut. 



Figure I2£ is nearly one-fourth the length of the embryo posterior 

 to figure I2d; it is approximately in the middle region. The diam- 

 eter of the embryo has been gradually decreasing until now it is 

 very much less than in the head region. The section being behind 

 the head-fold the entoderm (en) is nearly flat and the enteron is 



