Herrick, Brain of the Snake. 169 



(7.) The isthmus lias been greatly disturbed by the forma- 

 tion of the cerebellum and pons. The interpeduncular body 

 represents a rudiment which may throw light on the apparent 

 absence of nerves from this important segment. 



The study of the black snake and garter snake embryos, 

 while not completed, suggests the following statements : 



(i.) The first embryonic vesicle suffers almost complete 

 rotation about the hypophysis (infundibulum.) 



■ (2.) The mouth of vertebrates is terminal or sub-terminal, 

 the stomatodeum being morphologically the modified skin of a 

 dorsal region of the head. 



(3.) The chordal part of the brain represents its entire 

 length, the so-called prechordal being but revolved dorsal pro- 

 jections. 



(4.) Premature closing of the nerve tube due to cephaliza- 

 tion has served to retain the homologues of certain ganglia in 

 the medullary tube. 



(5. ) The formation of the primary optic vesicle is a bel- 

 ated separation of such a pair of ganglia. 



(6.) The fibres of the special sense nerves may retain the 

 habit of ordinary sensory fibres of producing bifurcating collat- 

 erals. 



(7.) If neuromeres once existed in the fore-brain they 

 would only be visible at an early stage and would be obscured 

 by the altered conditions. The so-called fore-brain neuromeres 

 differ from those of the medulla and cord in involving only dor- 

 sal structures. They are wholly illusory from a morphological 

 point of view. 



(8) The position of the optic recesses is morphologically 

 secondary and must not be given a too prominent morpholog- 

 ical rank. 



(q) The commissures (prse , callosal, fornix, habena, 

 supra- and post-,) are similar and belong to the dorsal system. 

 They are interrupted by the complicated system- of dorsal 

 diverticles. 



(10) Two subdivisions occur in the base of the mesenceph- 

 lon at an early stage but they do not correspond to the 3d and 

 4th nerve roots, both of which lie in the caudal member. 



