NOTES ON ELASMOBRANCH DEVELOPMENT. 577 
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 structure of the adult 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 
ofstructure 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 
