466 B. F. KINGSBURY 
Furthermore, neither the brain nor the head can be considered 
by itself developmentally or for that matter phylogenetically 
and any adequate interpretation of the developmental plan of 
the head and brain must be in full harmony with the develop- 
mental pattern of the body as a whole. Also I venture to add it 
should be appreciated that in as much as all vertebrates conform 
in the fundamental structural plan of the adult body, there must 
likewise be conformity in the developmental plan. In other 
words, the differential growth of all vertebrates must of very 
necessity follow the same pattern. The full significance of this 
is I think frequently overlooked. It is quite essential that the 
developmental pattern be established as well as the develop- 
mental processes ascertained. 
Without considering structural aspects purely neurological 
wherein the interpretation ‘D’ would clarify difficulties, there 
are certain general questions of importance in the choice of 
interpretations that may be answered by an appeal to the facts 
and which the present paper aims to consider. These are: @) 
The determination of the cephalic end of the primitive brain 
plate. Johnston, followed by Kingsbury it will be recalled, 
included the chiasma; His terminated it at the infundibular 
recess, Schulte and Tilney at the mammillary recess. The 
existence of the primitive optic furrow of Johnston. 6) The 
question as to the existence and extent of a sutura terminalis as 
distinct from a sutura dorsalis. c) The significance of the 
sutura neurochordalis of His and the primitive extent of the 
floor plate. In the examination of these specific questions the 
general significance of conception ‘D’ of figures 1 and 2 will come 
out in clearer outline. 
Material. Inthe choice of forms in which to test the questions, 
two were selected for consideration in this paper—the shark 
(Squalus acanthias) and the chick. Important considerations 
here were: a) the availability, since it is quite important that a 
close series of stages at critical epochs be at hand; b) ease of 
orientation. Medial sagittal sections through the head are quite 
essential. Model reconstructions from transverse series alone 
are far less useful and trustworthy. A considerable number of 
