Herrick, Brain of the Snake. 171 



In general, the medulla may be divided into three portions: 

 The isthmus, or that narrowed portion which lies cephadad of 

 the trigeminus; second, the expanded portion including the neu- 

 romeres of the fifth, seventh and eighth nerves; third, the re- 

 maining neuromeres which grade into the spinal portion. There 

 is some evidence that the isthmus contains two neuromeres, the 

 expanded portion contains three and the remainder two obvious 

 expansions. 



The neuromeres are well seen in Plate XVII, Figs. 1-6, and 

 in the sections from black snake embryos. Plate XIX, Figs. 4-5. 



The figures of Plate XVII show the extended roof of the 

 diencephalon at this age, also the great development of the 

 optic and infundibular recesses. There is a recess at the cite of 

 the future mammillaria, as in fishes. Tliere is a thickening in 

 the region of the pes where rapid proliferation is going on. 

 This is obviously preparing for the extensive anastomoses de- 

 scribed in a previous paper. 



The floor of the mesencephalon is obviously composed of 

 two portions but evidence is wanting to sh(jw that there are dis- 

 tinct neuromeres corresponding to the third and fourth nerves. 



The first neuromere (isthmus) of the medulla is about twice 

 the length of the others and from its roof the cerebellum is de- 

 veloping. The trigiminal neuromere is greatly expanded but is 

 not longer than others. The third expansion has no root at this 

 stage. It is not unreasonable to suppose that the trigiminal has 

 gathered up roots from the neuromeres cephalad and caudad. It 

 would thus represent three neuromeres, while the sixth could be 

 looked upon as a remnant weak and small. 



The fourth expansion receives the eighth from the anlag on 

 the' auditory sac and gives rise to the seventh, while the fifth 

 and sixth neuromeres are as usual. Fig. 4, of Plate XIX indi- 

 cates that this neuromerism is even more prominent in the cord 

 than in the medulla. It seems impossible that these remarkable 

 and regular changes could be due to mechanical effects of the 

 mesoderm segments. 



The Origin of the Tuber Olfactorii. — Plate XV, Figs. 

 9- II gives quite a clear idea of the origin of the olfactory struc- 

 tures. As the epithelium begins to invaginate the proliferation 

 increases, neuroblasts accumulate on the side nearest the brain 



