The Development of the Alligator 291 
several layers of cells, so that it resembles to a 
considerable degree the spinal cord of the same 
region. In the narrow space between the spinal 
cord and the hindgut is seen the notochord (nd), 
somewhat flattened and relatively and actually 
smaller than in the preceding figure. A few scat- 
tered blood-vessels may be seen in the mesoblast at 
various places. 
A sagittal section of an embryo of this stage, 
drawn under the same magnification as were the 
transverse sections, is shown in Figure 13g. The 
embryo being bent laterally could not be cut by 
any one plane throughout its entire length, so 
that only the anterior end is represented in the 
figure. The amnion (a) may be clearly seen except 
at certain places where it is closely adherent to 
the superficial ectoderm. Under the low magni- 
fication used the superficial ectoderm cannot be 
distinguished from the ectoderm of the nervous 
system. The plane of the section being in the 
anterior end almost exactly median, this part of 
the central nervous system is seen as the usual 
retort-shaped cavity, while in the region back 
of the brain, where the neural canal is narrow, the 
section passes through the wall of the spinal cord 
(sc) and does not show the neural canal at all. 
The wall of the forebrain (fb) is quite thick, espe- 
cially at the extreme anterior end; the wall of the 
midbrain (mb), where the marked cranial flexure 
takes place, is somewhat thinner, and it gradually 
