186 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
from the expansions of the myelon? Every fact which we possess seems 
to me to argue against their primitive nature. In my opinion the 
assumption of Herrick (*92), that, “if neuromeres once existed in the 
forebrain, they would be visible only at an early stage, and would be 
obscured by altered conditions,” is the more reasonable of the two 
assumptions. On the basis of structure and of relation to other seg- 
mentally arranged organs, however, I conclude that the primary vesicles, 
the forebrain and midbrain, give evidence — as do the primary expansions 
of the hindbrain — of the primitive segmentation of the Vertebrate head. 
I now turn to an examination of these relations, first, to those of neuro- 
meres and somites, since they are the most important. 
V. The Relation of Neuromeres to Somites. 
a. RELATION OF MYELOMERES TO SOMITES. 
Since the myelomeres, as has been stated, show a definite (numerical) 
correspondence with the trunk somites, the expansions of the spinal cord 
alternating with the somites, it is evident that proof of the serial homol- 
ogy of myelomeres and encephalomeres will rest very largely on the dem- 
onstration of a similar correspondence of the latter with head somites, if 
there be such. Yet, so far as I know, Miss’ Platt is the only investigator 
who has affirmed that there is such correspondence for the head region. 
She writes (91, p. 82) as follows: “The line of somites [in Squalus] 
alternating with the neuromeres is continued into the head as far for- 
wards as the alimentary pocket which is to form the second visceral cleft. 
Here complete divisions of the mesoderm cease, but serial depressions 
in its dorsal wall indicate incomplete divisions into three parts above 
the hyoid arch (van Wijhe found two somites here) and two parts above 
the mandibular arch [van Wijhe found one somite here]. Like the 
somites of the trunk, the divisions thus marked off alternate with 
the neuromeres, lying opposite successive constrictions of the brain. 
The anterior division of the mandibular cavity corresponds to the con- 
striction that separates the midbrain from the hindbrain, or to that from 
which the trochlear nerve arises.” The same investigator likewise says 
in regard to Necturus (’94, pp. 960, 961): “ Hinter der Hyomandibular- 
spalte wechseln die primitiven Neural- und Mesoderm-Segmente regel- 
miissig mit einander ab. Die mesentodermale Segmentation ist dieselbe, 
die von v. Wijhe den Selachiern zugeschrieben wird.” 1 
1 It is seen that Miss Platt finds the segmentation, both neuromeris and meso- 
meric, different in Squalus and Necturus. While in embryos of the former she 
