502 LOCY. [Vol. XI. 



the Croonian Lectures, in 1869, by Huxley, in which he com- 

 pletely overthrew the vertebral theory of the skull, withdraw- 

 ing attention from the superficial sutures in the cranium and 

 directing it to the cranial nerves and branchiae as bearing evi- 

 dence to the segmentation of the head. Gegenbaur, in 1872, 

 studied the cranial nerves especially in relation to the branchial 

 clefts and reached the conclusion that there are nine segments 

 represented in the head. Another distinct advance was made 

 by Balfour, who first studied the segmental divisions of the 

 mesoblast in the head of the Elasmobranchs, and identified by 

 this means eight head-somites clearly represented. He also 

 expressed the conviction that there were primitively a larger 

 number of segments but, owing to extreme modifications of 

 the head region, they are no longer clearly represented. This 

 was at bottom the same problem, but it was now shifted upon 

 organs that are truly segmental. 



From the time of its discovery, this segmental division of 

 the mesoblast in the head became a great favorite with mor- 

 phologists in elucidating the problem of head segmentation. 

 The mesoblastic divisions seemed, so far as the evidence went, 

 to embody the most direct survivals of the original segmenta- 

 tion and, therefore, to be the most promising line along which 

 to work out the problem. Valuable contributions have been 

 made along this line since Balfour's time, by Marshall, Van 

 Wijhe, Dohrn, Killian, Oppel, and others, and the myotomes 

 of the head have continued to hold their high position in the 

 minds of morphologists as the most significant remnants of the 

 original segmentation. 



Notwithstanding all these researches the original problem is 

 still unsolved. There is no agreement as to the number or the 

 nature of the primitive segments, and about the only point that 

 may be regarded as settled beyond controversy is that the head 

 and brain were primitively divided into segments. 



Recently, there has been added as a factor in the discussion, 

 observations on segmental divisions of the neural tube. 

 Althouo;h such divisions have attracted the attention of several 

 observers, their importance in the problem of cranial segmen- 

 tation has not been appreciated. They have been regarded as 



