DEVELOPMENT OF THE AMERICAN ALLIGATOR — REESE 33 



nous with the wall of the spinal cord (sc). The ectoderm (ec) is 

 thicker than ever, except in the median plane, where it passes over 

 the spinal cord. The mesoblast fs more abundant than in the pre- 

 ceding figure, and shows on the left what appears to be a distinct 

 myocoel (inyc), though in surface view the mesoblastic somites do 

 not extend this far toward the tail. 



Stage X 

 Figures 13-132 (Plates XIV, XV, XVI) 



This embryo (fig. 13) is about 5 mm. in length, and hence is 

 slightly smaller than the preceding stage, though somewhat more 

 advanced in development. The medullary canal is still apparently 

 unclosed for a short distance at the extreme posterior end ; this ap- 

 pearance is probably due to the neurenteric canal (nc) and to the 

 thinness of the roof of the medullary canal rather than to any lack 

 of fusion of the medullary folds. The optic vesicle is more distinct 

 than in the preceding stage ; a somewhat similar, though smaller, 

 opacity (0) marks the position of the ear. There are now about 

 twenty pairs of somites, though it is difficult to determine their exact 

 number on account of the torsion of the body. The amnion is at 

 about the same stage of development as in stage ix. The heart 

 (ht) is a large double mass, whose outlines may be dimly seen when 

 the embryo is viewed by transmitted light. The vitelline vessels 

 (vv) are still but faintly outlined in the vascular area; the veins and 

 arteries cannot yet be distinguished from each other. The gill 

 clefts, though not visible externally in the embryo drawn, may be 

 seen in sections of this stage as evaginations of the wall of the 

 pharynx. 



The transverse sections of this stage are slightly more advanced 

 in development than was the embryo that has just been described in 

 surface view. Only those sections have been figured which show a 

 decided advance in the development of some special structures over 

 their condition in the preceding stage. The sections of the pre- 

 ceding stages were drawn under a magnification of eighty-seven 

 diameters ; those of this and the following stage were drawn under 

 a magnification of only forty-one diameters. All of the figures have 

 been reduced one-half in reproduction. 



Figure 13a is the most anterior section of this series to be de- 

 scribed. On account of the cranial flexure, which causes the long 

 axis of the forebrain to lie at right angles to that of the spinal cord, 

 this section cuts the head region longitudinally. The ectoderm (ec) 

 3— al 



