430 CHARLES HILL, 



prosencephalon cephalad to his neuromere 1, from which I infer he 

 does not include this portion in the true segmental region. 



If my interpretation of McClure's position is correct, it seems 

 to me he has described the conditions that are represented in my 

 Figs. 40 and 41. At this stage the thalamencephalou is clearly divided 

 into 2 segments by the presence of a dorso-lateral invagination (s) 

 in which the fibres of the posterior commissure develop. The fact 

 that this invagination begins as a dorsal constriction, and that its 

 appearance is ontogenetically late, after much specialization has oc- 

 curred and long after the other segmental constrictions have appeared, 

 is strong evidence that this invagination has a very different morpho- 

 logical value than the constrictions of true encephalic segments. 



Regarding the presence of 2 neuromeres in the mid-brain of 

 chick embryos during the 3. day of incubation, McClure admits 

 that the evidence is very problematic. The mid-brain, he says (p. 48), 

 "has the appearance of being an enlarged neuromere". . . . "Its cell 

 structure is radial but its nuclear arrangement does not conform to 

 a typical neuromere except that at its anterior and posterior limits 

 the cells are crowded together and do not enter the adjoining structure." 

 In conclusion he writes (p. 49) : "Taking into consideration the size 

 of the mid-brain neuromere in comparison with the remaining neuro- 

 meres of the brain as well as its neuromeric characteristics, also 

 the fact that two nerves arise from it which are probably either two 

 segental nerves or parts of the same, also the investigations of Kupffer, 

 previously mentioned, in which he states that he found at least eight 

 segments in the hind- and mid-brains of the Trout and Salamander, 

 there can be little doubt left but that the mid-brain originally consisted 

 of at least two neuromeres and that in all probability the 3. and 4. 

 nerves were segmental nerves of these neuromeres respectively." 



I have studied the mid-brain of chick embryos with much care 

 by the method of sections and dissection. In embryos 30 hours 

 and older I find no evidence of segmental division excepting the 

 presence of the oculomotor and trochlear nerves whose fibres connect 

 with the mid-brain. In young embryos, however, transverse grooves 

 divide the mid-brain into 2 segments which, according to the 

 descriptive portion of this contribution, constitute encephalic segments 

 4 and 5 (Figs. 2ö— 30). 



Regarding the 13 encephaloraeres that Zimmermann, '91, affirms 

 of embryos of all higher Vertebrates, I have but little to say, as the 

 author in his brief paper does not state the criteria used in deter- 



