444 Julia B. Platt 
my surprise both Froriep ('94) and SEWERTZOFF (95) understand 
me to imply that the head of Necturus contains 9 segments. I 
regret this misunderstanding the more since we read FRorIEr's 
reviews with no less delight in their clearness, than confidence in 
their impartial accuracy. 
In reviewing my paper on Acanthias, Froriep (91 0, page 585) 
also stated, but with better ground for the belief, that I found 
12 segments in the head of Acanthias. In actual fact, however, 
I had not studied the occipital region in Acanthias, and did not 
know how many segments enter the head, nor have I at any time 
made any statement of belief regarding the number of seg- 
ments that enter the head of any Vertebrate. My first study 
on the segments taken into the occipital region of the head is 
presented in this paper, and I trust it will appear to those who 
read the following pages that while I believe it to a certain extent 
possible to homologize segment with segment in comparing different 
Vertebrates, I also believe it quite possible that one or more somites 
have dropped out in the region immediately posterior to the ear, in 
connection with the reduction of the visceral arches, which would 
make the number of primitive segments included between the first 
segment of the head and the last, still undetermined. 
Much difference of opinion exists regarding the serial homology 
of the prae- and postotic divisions of the axial mesoderm. While 
v. WiJHE believes that the somites he described are serially homo- 
logous structures, FRORIEP (85, ’87), not doubting the existence of 
the nine somites in the Selachian head, maintains that the anterior 
and posterior somites are not of equal value, and calls attention to 
the fact that a break exists in the middle of the row, where anterior 
as well as posterior elements may have been lost. In a brilliant 
critical review, GEGENBAUR (’87) also expresses his belief that the 
anterior somites of the head are not homodynamous with the occi- 
pital. Welcoming the continuous segmentation described by v. WIJHE 
as a valuable confirmation of his belief in the segmented structure 
of the Vertebrate head, GEGENBAUR regards the first 6 of v. W1JHE’s 
somites as belonging to the primitive head, but believes that the 
3 posterior somites described by v. WısHe, have migrated into the 
head from the trunk. Finally, RABL ('89) takes the extreme view 
that segmental value can be assigned only to the postotic divisions 
of the axial mesoderm. 
I pause for a moment to comment on a statement by which 
