DEVELOPMENT OF THE AMERICAN ALLIGATOR — REESE 45 



tail, in close connection with the Wolffian bodies. At intervals they 

 are connected by narrow channels with the dorsal aorta ; no such 

 connection was present in the section drawn. The notochord (nt), 

 spinal cord (sc), muscle plates (nip), and spinal ganglia (sg) need 

 no special mention. The mesoblast is beginning to condense in the 

 neighborhood of the notochord, and the ectoderm is slightly thick- 

 ened laterally and dorsally. 



Figure 16c is in the region of the liver and the Wolffian bodies ; 

 it also shows the tip of the ventricular end of the heart. The liver 

 (/•/) is a large irregular mass, of a blotchy appearance under this 

 magnification, lying between the heart (vn) and the intestine (i). 

 Under greater magnification it is seen to be made up of indefinite 

 strings of cells ; and its still wide opening into the intestine may be 

 seen in more posterior sections. The intestine (i), which in this 

 section might be called the stomach, is a fairly large cavity with 

 the usual thick entodermic walls ; it is supported by a comparatively 

 narrow mesentery. The body cavity on the side next this mesentery 

 has the same thick lining that was noted in the region of the lungs. 

 The convolutions of the thick peritoneal lining may easily be mis- 

 taken in places for parts of the enteron. The Wolffian bodies may 

 be seen as two groups of tubules (wt) in their usual location. The 

 heart is cut through the ventricle (zw),as has been said. The section 

 being at right angles to the long axes of the villi-like growths of 

 the myocardium, the depressions between these mesoblastic cords are 

 seen as a number of small irregular areas, each one lined with its 

 endocardium. The incompleteness of the body wall below the heart 

 is apparently clue to an artificial break and not to a lack of fusion. 

 The only point that need be mentioned in connection with the struc- 

 tures of the dorsal part of the section is that the distinctness of the 

 myocoel (myc) on the right side is somewhat exaggerated. 



Figure 16/ is in the middle region of the embryo, where both 

 spanchnopleure and somatopleure are unfused. Owing chiefly to 

 the unclosed condition of the midgut (i) and to the increase in 

 length of the mesentery (ins), the section is quite deep dorso- 

 ventrally. The continuation of the amnion (a) with the somato- 

 pleure is of course here evident. 



The most striking feature of the section is the marked projection 

 of the Wolffian ridges, though no local enlargements of these ridges 

 indicate the rudiments of the limbs. A large mass of Wolffian 

 tubules (zvt) is seen projecting into the upper part of the body 

 cavity on each side ; close to each of these masses is the posterior 

 cardinal vein (pc), and between them is the large aorta (ao). The 

 other structures are about as in the preceding section. 



