214 



Zweite allgemeine Sitzung. 



M esodermal Somites. — Before and during the same 

 period attempts were made to prove that the skull had a verte- 

 bral structure. The early fanciful schemes of Goethe, Oken, 

 and others received a great deal of support until the destructive 

 criticism of H u x 1 e y rendered them untenable. Then Gegenbaur 

 brought forward a masterly resuscitation of the old conception. 

 And since that time modern work in embryology has established 

 the fact that a certain part at least of the hinder portion of the 

 head is developed in a manner identical to the vertebral region. 

 It is now well known that a number of somites are always involved 

 in the formation of the head, and attempts have been made with 

 ref erence to these and the so-called head cavities to state (as v a n 



Streak blastoere 





notochord i 



Segments 



Figs. 2 and 3. 



Obliquely sagittal 



sections of the same 



embryo. 



The positions of the 



sections are indi- 



cated by lmes in 



fig. 1. 



Segments notochord 



W i j h e and also Z i e g 1 e r have done f or example) the number 

 of segments included in the cranium. 



Segmentation of the Head. — What I have 

 done is to combine the results obtained by these two lines of 

 enquiry. The somites as has been said alternate with the neuro- 

 meres. But more somites than are required appear during the 

 early development of all Craniates, and they suffer regression 

 in an antero-posterior direction. In a third day Gull embryo there 

 are seven segments included within the head. These as usual 

 alternate with the neuromeres, the first being opposite the interval 

 between the 6th and 7th rhombomeres. This clearly suggests 

 that the number of rhombomeres is 13. It is true the seven seg- 

 ments are speedily reduced to the typical two or three of the 

 hypoglossal region, but their temporary presence is sufficient 

 to Warrant the conclusion that the glossopharyngeal and vagus 

 region was originally segmented. 



In my previous papers I was led to conclude from this con- 

 sideiation alone that the whole of the chordal area of the head 



