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 (it) 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 
