76 PROFESSOR A. MILNES MARSHALL. 
notochord and the pituitary body. As seen in the figure, the no- 
tochord (z.) as it runs forward beneath the hind brain tapers 
considerably. On reaching the level of the head cavity it 
becomes still more constricted; it also leaves the floor of the 
hind brain, becomes curiously twisted on itself, and, after running 
forward a short distance in the mesoblast, bends sharply down- 
wards and backwards, its terminal portion being very closely 
applied to the dorsal wall of the head cavity. The figure also 
shows that this median portion of the head cavity is in very 
close relation both to the outgrowth from the fore brain to 
form the infundibulum (af), and to the pituitary involution 
from the mouth (pit.) 
At stage these relations remain unchanged, the sole difference 
being that the two walls of the head cavity have come in contact 
so as to obliterate the lumen. By stage o the notochord has 
lost its connection with the head cavity and now runs straight 
forwards to its termination, while the walls of the head cavity are 
reduced to a very thin cellular plate, lying in close contact with 
the infundibulum and pituitary involution. I have not followed 
the fate of this median cavity any further, and have not deter- 
mined whether it forms any part of the adult pituitary body, 
though, from its position, this would appear not improbable. 
In fig. 23 this median portion of the first head cavity (J.) is 
shown in horizontal section at a stage intermediate between L 
and very shortly before its obliteration. 
The premandibular cavity itself at stages k and 1 “ forms a 
space of considerable size, with epithelial walls of somewhat 
short columnar cells.” Its position and relations are well shown 
in fig. 4, and in the series of figs. 8 to 14. It lies, as shown in 
figs. 4 and 8, very close to the posterior surface of the eye (0. v.). 
During stage m it becomes still more closely applied to the eye ; 
it becomes partially doubled up on itself so as to form a hollow 
cup, which closely invests the eye on its posterior and inner sur- 
faces, as shown in figs. 18 and 20 (1), and figs. 34 and 35. 
From the walls of this cup, as will be shown later on, certain of 
the eye muscles are developed. 
This premandibular cavity presents at certain stages a marked 
constriction at its ventral part, tending to partially divide the 
cavity into two; this is shown at stage 1 in fig. 9; and still more 
clearly at a stage intermediate between ~ and m in fig. 21. 
Whether this indicates an aborted division of this cavity into 
two parts, each equivalent to one of the posterior cavities, or 
merely a division into dorsal and ventral portions such as occurs 
in the hinder cavities, J have been unable to determine. 
The premandibular cavity persists very much longer than any 
1 Balfour, op. cit., p. 206. 
