NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 149 
left to van Wijhe (’82) to demonstrate in Selachian embryos an uninter- 
rupted continuity, and a direct morphological comparability, of head 
and trunk “ Mesodermsegmente,” and thus, in the opinion of many 
morphologists, the existence of an “acraniote” stage in the development 
of craniote embryos. Since the “mesodermal segments” or somites 
were regarded as the best evidence of the primitive segmentation, it 
was at first believed that the problem of the morphology of the Ver- 
‚ebrate head, as regards both number and nature of segments, had at 
ast been solved by van Wijbe. His conclusion was that nine seg- 
ments, four of which were pre-otic and five post-otic, enter into the 
formation of the Vertebrate head, or at least the Selachian head. 
Yet one who studies the literature of the decade and a half that has 
elapsed since van Wijhe wrote his famous paper must conclude, from 
the great divergence of opinion which still prevails among the most 
competent investigators as regards both nature and number of head 
segments, that the problem is “noch nicht aus der Welt geschafft.” 
According to Froriep, Kastschenko, and Rabl, the segments of the pre- 
otic and post-otic regions are of a fundamentally different kind. Fur- 
thermore, while Rabl (92) finds not over three segments in the entire 
pre-otic region, Dohrn (90) finds in the same region twelve to fifteen 
segments, serially homologous with trunk segments. These, indeed, 
represent extremes of opinion, for the majority of morphologists agree 
with Gegenbaur and van Wijhe that pre-otic segments are few but com- 
parable with trunk segments. The chief causes of the present dis- 
agreement of morphologists are two. In the eager search for evidence 
of segments investigators have often failed (1) to control their results, 
based upon the study of a single organ system, by a comparison of the 
actual conditions which obtain in other organ systems in the same 
organism ; and (2) to control conclusions based upon a single organ- 
ism by appeal to the facts and conclusions of comparative anatomy and 
embryology. As the result of the healthful scepticism of such accurate 
observers as Froriep, Kastschenko, and Rabl, the necessity for such con- 
trol now seems too obvious to need repetition here. 
While morphologists (excepting Gegenbaur) in attempting to elu- 
cidate the problem of cephalic segmentation have based their con- 
clusions chiefly on the study of the mesodermal segments, — since these 
have seemed to afford the best criteria of segmentation, — yet other 
embryonic structures have also been studied, viz. the segments of the 
central nervous system, or “neuromeres,” the nerves, the epibranchial 
organs, the blood-vessels, and the visceral arches. 
