186 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



from the expansions of the myelou 1 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 au 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 Mtelomeres 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 Piatt 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] 

 altei'nating 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 IS'eural- und Mesoderm-Segmente regel- 

 massig mit einander ab. Die mesentodermale Segmentation ist dieselbe, 

 die von v. Wijhe den Selachiern zugeschrieben wird."^ 



1 It is seen that Miss Piatt finds tlie segmentation, both neuromeric and nieso- 

 meric, different in Squalus and Necturus. \Yhile in embryos of the former she 



