The Development of the Alligator 283 



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 Qit) 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 preceding 

 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 described. 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) is of varying thickness, the 



