42 McCLURE. [Vol. IV. 



The Relation of the Myclonieres to the Mesoblastic Somites. 



The Myelomeres are intersomitic ; that is, the centre of each 

 Myelomere is opposite the space between two somites (Figs, i, 

 2 and 3). The dorsal branches of the spinal nerves pass from 

 the external surface of the Myelomeres to the space between two 

 somites, which is opposite their point of origin, and fuse with 

 the ^piblastic thickenings to form the spinal Ganglia. 



Comparative Structure of the Neuromeres. 



In the hind-brain of the lizard and chick six neuromeres 

 are distinctly seen. Figs. 5, ^a, 6, and 6a, which in each case are 

 of exactly the same size with the exception of the Vagus Neu- 

 romere (Nm X.) which is slightly longer than the others. In 

 Amd/ystoma, Figs, 4, 4a, only five neuromeres are found in the 

 hind-brain. The Abducens Neuromere (Nm VI.) is not present. 

 The remaining neuromeres are of equal length except the 

 Vagus Neuromere {Nm X.) and the Trigeminal Neuromere 

 {N'jn v.), which are somewhat longer than the others. We have 

 already seen that the Vagus Neuromere in the lizard and chick 

 is somewhat longer than the others, but that the Trigeminal 

 Neuromere does not vary. In Ainblystovia, the Trigeminal 

 Neuromere is equal in length to about two of the three remain- 

 ing neuromeres of the average dimensions. (Figs. 4, 4a) 



This variation in size of the Trigeminal Neuromere is due in 

 all probability to the coalescing of the Abducens Neuromere 

 with the Trigeminal Neuromere to form one neuromere. The 

 fact that the recent Amphibia are somewhat removed from the 

 main vertebrate line, and that their development has been 

 influenced by the great quantities of food yolk present, may 

 account in some degree for the varying structure of the Tri- 

 geminal Neuromere in Amblystom.a. 



So much for the similarity and points of difference which 

 exist between the neuromeres of the medulla of A?iolis, Avi- 

 blystoma and the chick, so far as their relations of size are con- 

 cerned. Now in regard to their histological structure, I find 

 that the four characteristics given by Orr for the neuromeres 

 of the medulla of Anolis are represented in every respect 

 in the structure of the neuromeres of the medulla 

 of Amblystoma and the chick; that is, the cell arrangement of 



