DEVELOPMENT OF THE AMERICAN ALLIGATOR — REESE 29 



About seventeen pairs of somites are present. 



The amnion extends over the anterior two-thirds of the embryo. 



The above-mentioned increase in the diameter of this embryo 

 over that of the preceding is evident when the first two transverse 

 sections of this stage are compared with the corresponding sections 

 of the earlier stage ; in the middle and posterior regions there is not 

 very much difference in size. 



Figure 12a passes through the region of the forebrain. This end 

 of the embryo lies on its side, as was noted above and as may be 

 recognized from the relative positions of the head and the overlying 

 yolk (3/). The great size of this and the following figure is due 

 partly to the increase in size mentioned above and partly to the fact 

 that the sections pass through the region of cranial flexure. The 

 present figure (12a) represents the brain cavity as large and dumb- 

 bell-shaped, with comparatively thick walls of compactly arranged 

 cells. The ventral end of this cavity (fb) is cut anterior to the 

 region of the optic vesicles, while the dorsal end (nib) may perhaps 

 be called the midbrain. In the sections that follow this one the two 

 cavities are distinct from each other. The medullary canal, as was 

 stated above, is now completely enclosed, except for the ventral 

 opening of the neurenteric canal, to be presently noticed. Sur- 

 rounding the brain is a considerable mass of mesoblast (mcs). It 

 is composed of the typical stellate cells. The ectoderm (ec) is made 

 up of the same irregularly and loosely arranged cells that have been 

 seen in earlier stages ; it is of unequal thickness in different regions, 

 the thicker parts being at the sides. The amnion (a) has the usual 

 appearance, and in this region of course completely surrounds the 

 embryo. 



Figure 12b is ten sections posterior to the section just described. 

 The width of the embryo is greater in this region, but the dorso- 

 ventral diameter is about the same as in the more anterior section. 



The overlying yolk and blastoderm are not shown in any figure of 

 the series except the first. In this figure the forebrain (fb) and 

 midbrain (nib) are widely separated instead of being connected, as 

 in the preceding figure, where the section passed through the actual 

 bend of the cranial flexure. The anterior and ventral part of the 

 cranial cavity, the forebrain (fb), is nearly circular in outline. It 

 exhibits on one side a well-marked optic vesicle (ov), which is suffi- 

 ciently advanced in development to show a rudimentary optic stalk. 

 The outer wall of the optic vesicle is in close contact with the super- 

 ficial ectoderm, which shows as yet no sign of the formation of a 

 lens vesicle. The plane of the section being probably not quite at 

 right angles to the long axis of the embryo, the optic vesicle of one 



