No. I.] 



THE PRIMITIVE VERTEBRATE BRAIN: 



41 



V. Trigefninal Neuromere. 



The first and most anterior neu- 

 romere of the hind-brain. 



VI. Abducens Neiiroviere. 



The second neuromere of the 

 hind-brain, absent in the Newt. 



VII. Facial Neiirotnere. 



The third neuromere of the hind- 

 brain. 



VIII. AHclitory IVeuromere. 



The fourth neuromere of the hind- 

 brain. 



IX. Glossopharyngeal Neuromere. 

 The fifth neuromere of the hind- 

 brain. 



X. Vagus Neuromere. 



The sixth neuromere of the hind- 

 brain. 



Comparative StrucUire of the Myelotneres. 



The spinal cord is of clearly segmental character, and at a 

 certain period of its embryonic development, at the time of for- 

 mation of the mesoblastic somites, we see that its lateral walls 

 are constricted in a manner similar to those of the encephalon, 

 and that the transition from the former to the latter is a gradual 

 one. 



Gross mounts of chick embryos ranging between 35 and 46 

 hours old clearly show this structure ; also Figs. 4, 4^, which 

 are longitudinal horizontal sections of Amblystonia. 



I find that the structure of the Myelomeres in the 

 Newt, Lizard and Chick, conforms in every respect 

 to the four characteristics which Orr gives as 

 found by him in the neuromeres of the hin d-b rain 

 of the Lizard. These four characteristics are quoted in full 

 on a previous page. 



An examination of Figs, i, 2, 3, all of which are camera draw- 

 ings of neuromeres of the spinal cord, shows — 



1. "That the neuromeres have the appearance of small arcs 

 of circles, i.e. one lateral half of each {Nni). And that the con- 

 strictions are exactly alike on each side of the brain." 



2. " That the cells are elongated and are placed radially to 

 the inner curved surface of the neuromere" (in). 



3. " The nuclei are generally nearer the outer surface {out), 

 and approach the inner surface {in) only towards the apex of the 

 ridge " {ap). 



4. " On the line between the apex of the internal ridge {ap) 

 and the pit of the external depression (ex) the cells of adjoining 

 neuromeres are crowded together, though the cells of one neu- 

 romere do not extend into another neuromere." 



