THE PRIMARY SEGMENT A TION. I 2 5 



conviction that there were primitively a larger number of seg- 

 ments, but, owing to extreme modifications of the head-region, 

 they are no longer clearly represented. 



From this time onwards, the myotomes became a great 

 favorite with morphologists in elucidating the problem of head 

 segmentation. They seemed, so far as the evidence went, to 

 embody the most direct survivals of the original segmentation, 

 and therefore to be the most promising line along which to 

 work out the problem. Van Wijhe's researches, published 

 about 1882, have been taken as the standard ones for reference ; 

 he identified nine head somites moulded in the mesoblast of the 

 head. This line of investigation received a great stimulus in 

 1890, from the work of Dohrn, who discovered eighteen or 

 nineteen myotomes in the head of torpedo embryos. In 1892 

 Killian substantiated his discoveries and reduced the enumer- 

 ation by one. It appears, therefore, that there is a larger 

 number of head somites than was at first supposed. 



The neural segments are comparatively recent in their claims 

 to attention as bearing evidence to the original segmentation 

 of the head, and their importance in this connection has not 

 been fully appreciated. Their early history has recently been 

 made known, l and this shows them in a new light. The neural 

 segments are the first to appear, and are less subject to modi- 

 fications in the early stages than the muscle segments of the 

 head. The large number of myotomes described by Dohrn 

 and Killian are transitory, and after a very brief existence they 

 become reduced by fusion, or absorption, or both, to the nine 

 head-cavities of Van Wijhe. But the neural segments make 

 their appearance very early and preserve their original number 

 and characteristics for a considerably longer period. It has 

 been assumed that the muscle segments are primary and that 

 the neural segments are secondarily moulded over them ; but, as 

 we shall see, this position cannot be sustained in the light of 

 recent observations, and it is timely to ask which set of seg- 

 mental structures affords the most reliable evidence as to the 

 primitive number of brain segments. 



1 Locy : Metameric Segmentation in Medullary Folds and Embryonic Rim. 

 Anat. Anz., Bd. IX, No. 13, 1894 ; Sojourn. Morph., Vol. XI, No. 3. 



