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- 

 tebrate head, as regards both number and nature of segments, had at 

 last been solved by van Wijhe. 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 Gegeubaur 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 Eabl, 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 " neuroraeres," the nerves, the epibranchial 

 organs, the blood-vessels, and the visceral arches. 



