Herri CK, Brain of the Snake. 175 



brain indicated by a dotted line as well as the boundaries r)f the several 

 compartments. 



PLATE XVIII.' 



Figs, 1-4. Four sections from a nearly horizontal series of the old- 

 est embryo of Eut;vnia (Plate XVI, Figs. 4, 5.) The mesencephalon is 

 obli(|uely cut and in Fig. 4 the roots of the oculo-motor (III) are in sec- 

 tion. The medulla neuromeres are numbered beyond the isthmus as fol- 

 lows: V, VI. VII, IX, X, in accordance with the chief nerve. The isth- 

 mus has no nerve, its roof being formed by tHe cerebellum. Max -|- 

 Maitd. The maxillary and mandiblar parts of the trunk of the trigem- 

 inus, the opthalmic being obviously distinct from the first. [208.] 



F/gs.j-g. Vive sections of the youngest Euteenia embroys [206]. 

 Parts numbered as before. The neuromerism is quite as prominent as 

 above. 



Fig. 13. [207.] A section along the mid-dorsal region, showing 

 that although mesomerism is complete there is no neuromerism, /. t'., al- 

 though the muscular segments are well-formed, the neural tube is not 

 segmented. 



Figs. 10, II. Two sections from the black-snake embryo ; chosen to 

 illustrate the substantial identity of neuromeric differentiation the two 

 groups. [263.] 



PLATE XIX. 



Fig. I. [207-5.] ""^ somewhat oblique frontal section through the 

 head of embryo of Eutajnia figured, Plate XVI, Figs, i, 2. [E. I.] The 

 section is so taken that the caudal part of the mesenceplialon is cut, and 

 a portion of the primary optic vesicle (Opt.) The existence of a distinct 

 sensory plate passing from the hypophysis about the protuberance of the 

 fore brain and including the lense capsule aniag is illustrated; the latter 

 point being shown in Fig. i, Plate XV. 



Fig. 2. [205-3.] Portion of the walls of the diencephalon of the 

 black-snake embryo figured in Plate XVI, Figs. 6, 7. The point from 

 which the drawing is taken is Fig. Sn, of the present plate. This figure 

 illustrates the elongation of the spongioblast (,f/), the dividing neuro- 

 blasts with karyokinetic figures (/■), the granule cells (gr.) which are re- 

 garded as indifferent nervous cells out of which neurons may be subse- 

 (juently formed, the neuroblasts transforming into neurons {>ii>.) and giv- 

 ing off axis cylinder* ((?. f.), in this case, caudad. 



