No. I.] THE PRIMITIVE VERTEBRATE BRAIN. 



43 



the neuromeres in the medulla of all three classes is the same. 

 It has already been shown that the structure of the Myelomeres, 

 in all three of the types studied, conforms in every respect to 

 the typical neuromere of the hind-brain. Thus we see that a 

 conformity of structure exists between the neuromeres of the 

 spinal cord and those of the hind-brain in the three forms studied. 

 By applying the description given for the structure of the 

 neuromeres in the spinal cord to Figs. 4.b, 4c; $d, and 6d, which 

 are camera drawings of the neuromeres in the medulla of 

 Aj)tblyst07na, Anolis, and the chick, it will be seen that the 

 structure of the neuromeres of the medulla and 

 spinal cord in the Amphibia, Reptilia and Aves is 

 identically the same. 



Comparative Structure of the Neurofneres of the Primitive 



Fore-brain. 



So far as known to myself, Orr was the first to notice the 

 presence of two neuromeres in the primitive fore-brain of the 

 Lizard, but he did not compare their cell-structure with that of 

 the neuromeres of the hind-brain. In addition to confirming the 

 presence of two neuromeres in the primitive fore-brain of the 

 Lizard, I have also found that the primitive fore-brain of the 

 Newt and Chick consists of two neuromeres. Also between 

 the mid-brain and optic neuromere {N^ti II.) of the Lizard, 

 Fig. d>a, there is a structure {Nm II.') which resembles a 

 portion of a neuromere. Its form is that of an arc of a circle, 

 but the radius of its arc is less than that of either of the two 

 remaining neuromeres of the primitive fore-brain, which I have 

 already said resemble arcs of circles, I make merely a passing 

 mention of this, for the reason that from the existing data noth- 

 ing but conjecture can result as to its neuromeric value; while 

 on the other hand if it is a neuromere, it ought to be present in 

 toto in some of the lower vertebrates. (See Appendix.) 



The fore-brain neuromeres of the Lizard and Chick, so far 

 as their external character and histology is concerned, are true 

 neuromeres. By external character I mean their form and 

 position with respect to each other. Figs. 8^, 9, illustrate the 

 following description of the neuromeres in the primitive fore- 

 brain of the Lizard and Chick. 



