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sophical morphology. Although there is no dissent upon the question 
of the segmental nature of the vertebrate head, the question of what 
constitutes the criteria of the segments, of what is their number and 
their anterior limit, is still under controversy. There has been a 
gradual change of front and the evidence has been shifted from sutural 
joints (OkEn 1807, GoETHE 1820, Owen 1832), to cranial nerves 
(HuxLEY 1858), to branchial clefts and cranial nerves (GEGENBAUR 
1872), head cavities (BALFOUR 1878, van WisHE 1882 and others), 
and finally to segments of the neural tube (BERANECK 1884, KUPFFER 
1886, Orr 1878, Mc CLurE 1890, ZIMMERMAN 1891, Waters 1892, 
Locy 1895, NeAL 1898 and others). 
The head cavities have in the past few years been great favorites 
with morphologists but there is greater disagreement among observers 
as regards facts of observation and as regards the interpretation of 
these structures than is generally supposed. In Elasmobranchs, the 
only group in which the developmental history of the cephalic meso- 
meres has been traced, there is no consensus of opinion as to their 
number, origin and morphological value. Rast ’92 reports 3 meso- 
meres in the pro-otic region while DoHrn finds 13 or 14, and many 
other variations in the number have been reported. SED@GwIck finds 
also a variable number in closely allied genera. VAN WIJHE, DOHRN, 
KırLıan, Miss PLATT, hold that the head cavities are all fundamental 
divisions of the cclomic cavity. GEGENBAUR, SEDGWICK, RABL, SE- 
WERTZOFF, maintain that some of them probably represent gill clefts 
and others, divisions of the ccelomic cavity. There is in addition to 
variation in number, such variable conditions reported in from size 
and histological conditions of these structures, that the mesoderm of 
the head appears to be an unfavorable structure for determining the 
primitive number and relationship of cephalic segments. GEGENBAUR, 
KASTSCHENKO, Rast. and SEDGWIcK, are disposed to discredit the 
morphological value of these segments, especially in front of the medulla. 
A review of the literature dealing with cephalic mesomeres makes 
two points clear: 
1) Observers disagree as to the number of pre-otic segments. 
2) They disagree as to their morphological value. 
A revival of interest in the problem of head segmentation came 
with the relatively recent discovery that the entire neural tube of 
young vertebrate embryos is divided by transverse constrictions into 
similar segments. There is better agreement among observers as to 
the number and character of these “neural segments” than tnere is 
as to the mesoblastic divisions of the head. There is substantial 
