The Development of the Alligator 319 
tiation than was seen in the preceding sections, 
though this cannot be shown under the low magni- 
fication used. The hindbrain (hb), which is cut 
near its anterior border, exhibits the usual mem- 
branous dorsal and thick ventral walls. The 
forebrain is here seen as three distinct cavities— 
a median third ventricle (tv), with a thick ventral 
wall, and a thin dorsal wall extended to form a large 
paraphysis (ep7), and two lateral ventricles (ch), 
the cavities of the cerebral hemispheres, whose 
walls are quite thick except on the side next the 
third ventricle. The sections of this series being 
slightly oblique, the eye is here cut on the right side 
only, where it is seen as a large, semicircular cavity 
(e) with thick, densewalls. Themesoblast, in which 
several blood-vessels (bv) are seen, exhibits three dis- 
tinctareas—a median, lighterzone, with amoredense 
area on either side. The significance of this varia- 
tion in the density of the mesoblast is not apparent. 
Figure 200 is only a few sections posterior to the 
section just described. It is drawn chiefly to show 
the appearance of the forebrain, the other struc- 
tures being about as in the preceding figure, except 
that both eyes (e) are here represented. The sec- 
tion passes through the wide opening between the 
third (tv) and the lateral ventricles (ch) and cuts the 
anterior edge of the pineal body' (epi). The 
t Subsequent investigation showed that the structure here described 
as the pineal body is, in reality, the paraphysis; the pineal body is 
absent in A. mississippiensis. 
