The Development of the AlHgator 319 



tiation than was seen in the preceding sections, 

 though this cannot be shown under the low magni- 

 fication used. The hindbrain {hh), 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 {epi), 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, dense walls. Themesoblast, in which 

 several blood-vessels {hv) are seen, exhibits three dis- 

 tinctareas — amedian, lighterzone, with a more dense 

 area on either side. The significance of this varia- 

 tion in the density of the mesoblast is not apparent. 

 Figure 2oh 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 



' 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. 



