252 ADAM SEDGWTOK. 



the disappearance of the primitive segmentation. This position 

 will be still further strengthened if my contention turns out 

 to be correct, viz. that in embryonic development the meso- 

 dermal cranial segments do largely become indistinguishable 

 before the adult landmarks have appeared. 



If my arguments and facts are sound, it follows that any 

 attempt to elucidate the adult structure of the head from the 

 point of view of its being composed of a series of segments 

 comparable to those of the trunk is foredoomed to failure ; 

 and the result of the whole inquiry shows up most thoroughly 

 the weakness of the position of those who hold embryological 

 research to be of small importance in comparison with the 

 study of adult structure. 



To a student of the multitudinous changes of structure which 

 an organism passes through in the course of its existence it 

 seems strange even now, and in the future will ever seem 

 stranger to the philosophical morphologist, that one condition 

 of structure only, and that the most complex and inexplicable, 

 should have been regarded by anyone as holding the key to 

 the solution of even a simple anatomical problem. 



To sum up the matter, v. Wyhe holds that there are nine 

 cranial segments which can be traced into the adult. Dohrn 

 holds that there is a much greater number of cranial somites, 

 some of which can be traced into the adult, and some of which 

 disappear. I agree with Dohrn in asserting that the anterior 

 mesoderm is completely segmented in Stage F, but maintain, 

 in opposition to him, that it is not possible to say how these 

 segments are related to adult structures, because they have 

 for the most part vanished before any of the adult landmarks 

 have appeared. 



The premandibular somite of Balfour (the first somite of 

 Wyhe). — There can be no doubt this is not, as Balfour sup- 

 posed, separated off from the mandibular. 



Kastschenko says that it develops from what he calls the 

 prechordal portion of the gut, which becomes solid when the 

 medullary plate is formed, and then subsequently again 

 acquires a cavity. I find myself unable to accept this account 



