44 McCLURE. [Vol. IV. 



1. One lateral half of each neuromere is an arc of a circle. 



2. The elongated cells are placed radially to the inner curved 

 surface of the neuromeres {hi). 



3. The nuclei are generally nearer the outer surface {put), and 

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

 ridge {ap). 



The arrangement of nuclei in the neuromeres of the primary 

 fore-brain does not always conform to the typical structure. 



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

 and the pit of the external depression {ex) the cells of the 

 adjoining neuromeres are crowded together, though the cells of 

 one neuromere do not extend into another. 



The fore-brain neuromeres of the Lizard and Chick persist 

 up to a certain stage in the embryo and finally disappear. 



The stage in which the fore-brain neuromeres of the Lizard 

 are fully developed is represented by Fig. 8a. In the Chick 

 these neuromeres are prominent in embryos from 36 to 96 

 hours old. The external character of the neuromeres in the 

 primitive fore-brain of the Newt is not found to be as perfectly 

 developed as those in the Lizard and Chick ; that is, each lateral 

 half of a neuromere does not form as perfect an arc of a circle 

 as in the latter, (Fig. 7). I am, however, in doubt whether this 

 variation from the general form is due to the fact that I did 

 not study the stages in which the neuromeres were most fully 

 developed, but rather those in which degeneration had already 

 begun but not been completed. In any case this was unavoid- 

 able, as the stages of this species which I possessed were limited 

 to a few. Possibly their development may have been arrested by 

 external means, due to the presence of yolk spherules, which 

 were found present in such great quantities, mixed in among 

 the cells, that it was a difficult task to make out the structure 

 of the neuromeres. It seems probable that one of the above- 

 mentioned reasons may explain this variation of form in the 

 fore-brain neuromeres of the Newt, But that these structures 

 are neuromeres or remains of neuromeres I think there can be 

 no doubt whatever, since their structure in most respects con- 

 forms to the typical structure. The cells have a radial arrange- 

 ment (Fig. 7), and between the neuromeres they are crowded 

 together, but the cells of one neuromere do not enter into 

 another neuromere. The arrangement of the nuclei is vari- 

 able and does not always conform to the typical one. 



