540 LOCV. [Vol. XI. 



the brain, there is to be added, according to McClurc's count, 

 four for the combined fore- and mid-brain, and, according to 

 Waters', five for these regions. Thus, McClure considers the 

 brain to represent a total of ten neuromeric segments, and 

 Waters makes a total of eleven segments. 



The European investigators, in general, have found a larger 

 number of these segments in the hind-brain, and usually have 

 not observed them in the fore-brain. Dohrn gives eight in 

 the bony fishes, Rabl seven or eight in the chick, Hoffmann 

 seven in Reptilia ; Froriep counted twelve in the entire brain 

 of mole embryos, and Zimmerman gives thirteen in front of 

 the vagus. The fore-part of the brain has been regarded as 

 non-metameric, and the question of the anterior extension of 

 segmentation is an important one. In regard to that point, the 

 evidence furnished by the neural segments, shows that the 

 segmentation extends throughout the fore-brain. This cor- 

 responds with the investigations of Whitman on the nervous 

 system of Clepsine, in which he shows conclusively that the 

 cerebral ganglia of that animal belong to the metameric series. 



Recent observations on a different set of segments, viz., 

 segmental divisions in the mesoderm of the head, have brought 

 to light a relatively large number of cephalic segments. Dohrn, 

 in 1890, observed eighteen or nineteen mesodermic segments in 

 the head of Torpedo marmorata of 3 mm. in length. Killian 

 ('9i) in the following year describes seventeen or eighteen in 

 Balfour stages /^and J oi Torpedo ocellata. He also makes a 

 correction to Dohrn's enumeration, making it correspond with 

 his own. In later stages of Selachians, as Van Wijhe has shown, 

 they are reduced to nine, or to ten according to Miss Piatt. 



There is such a fundamental relation between muscle and 

 nerve that we might expect myotomes and neural segments to 

 bear a direct numerical relation. My observations show a 

 smaller number of actual segments in the brain-walls than that 

 arrived at by Dohrn and Killian through studying the myotomes 

 of the head region. In the hind-brain I have found eight 

 segments represented in the earlier stages, and a ninth seg- 

 ment which, to all appearances, belongs to those of the spinal 

 cord, but later this ninth segment becomes clearly associated 



