FUNDAMENTAL PLAN OF VERTEBRATE BRAIN 479 
with full recognition that its use implies the acceptance of a 
line of blastoporal closure, actual or potential, with which it 
coincides, and which in the brain and spinal cord is marked by 
- the floor plate whose anterior limit I have placed at the fovea 
isthmi. 
Inasmuch as Johnston (09) and Schulte and Tilney (’15) did 
not consider the growth of the brain plate in its relation to the 
notochord and head mesoderm, the question of the neurochordal 
suture and its extent is not directly involved in the adjudication 
of their interpretations. Doubtless in prolonging a floor plate 
forward to the anterior end of the neural plate they but expressed 
an acceptance of the His analysis of the neural tube. 
It may be pointed out that the primitive furrow of the neural 
plate which appears in figures 6 to 11 and whose cephalic end is 
carried forward and down (figs. 11 and 12) and is lost to view with 
the development of the neural folds corresponds closely to the 
extent of the notochord. Figures 3 and 4, which are plottings 
to show the extent and relations of notochord mesoderm and 
entoderm in the embryos of figures 7 and 9, respectively, may be 
compared. This furrow I believe unquestionably corresponds 
to the neurochordal suture. 
Chick. While there exists in the chick as in the shark the 
same fundamental plan of relations of neural plate, notochord, 
prechordal plate and entoderm, with the characteristic morpho- 
logical transformations of forward growth and down-bending, 
it has not been possible clearly to determine the point of the fovea 
isthmi in the neural plate. It is obvious, however (figs. 27 to 32), 
that the notochord ends far from the anterior end of the neural 
plate separated from the infundibulum, as in the shark, by the 
extent of the prechordal plate, and at a level such as would, from 
the subsequent growth transformations in the brain, correspond 
closely with the level of the prospective fovea isthmi and the 
anterior end of the floor plate. The fovea isthmi and floor plate 
could not be detected, however, earlier than five days’ incuba- 
tion, and no serious attempt was made to bridge the interval by 
a detailed study of intervening stages. The essential similarity 
in the pattern in chick and shark may be seen by a comparison 
