126 BIOLOGICAL LECTURES. 



I have said that the metamerism of the head should be in- 

 vestio-ated in the light of work done on both myotomes and 

 neural segments, but here I can present only one side. The 

 case for the myotomes has been so completely presented by 

 Dohrn, Killian, and others that I waive a consideration of that 

 side of the question, and dwell on the history of the neural seg- 

 ments and speak of the new observations on them. 



The neural segments were observed in the hind-brain of 

 embryo chicks by Von Baer as long ago as 1828. They have 

 been observed and commented upon by many anatomists since 

 that time. In 1850 Dursy made the important suggestion 

 that the neural segments are genetically related to the cranial 

 nerves, but he had no direct evidence of this. To Beraneck 

 belongs the credit of having first demonstrated, in 1884, that 

 there is a definite relation between neural segments and certain 

 cranial nerves. This was the first substantial basis towards 

 establishing their segmental importance. Orr, "^J, McClure, 

 '90, and Waters, '92, followed this pioneer work, demonstrating 

 the definite relation ^ of the nerves of the hind-brain to specific 

 neuromeres. McClure showed also that the entire neural tube 

 is divided into similar segments, and Waters gave particular 

 attention to the fore- and mid-brain. He found in the lizard 

 evidence of three somites in the fore-brain, making a total of 

 eleven somites in the entire brain-region. 



In Europe Froriep, Kupffer, Rabl, Hoffmann, Zimmerman, 

 and others have made recent contributions to the knowledge 

 of these neural segments. In 1892 Froriep described anew 

 the neural segments and attached no particular importance to 

 them. He expressed the conclusion that the neural segments 

 are secondarily moulded over the segmental divisions of the 

 mesoblast. The latter, in common with most other writers, he 

 regards as primary. I hope to show you before the conclusion 

 of this lecture that the position cannot be sustained. 



Let us now see what may be learned regarding the primitive 

 segments of the head by the examination of young embryos. 



1 I prefer to omit in this general lecture the question of the relationship of par- 

 ticular cranial nerves to particular neural segments, which is of so great interest 

 and importance to the morphologist. 



