202 
J. D. F. GILCHRIST. 
The functional significance of the whole process seems very 
evident. A very little of the yolk is necessarily used up in 
the formation of the thin endodermal layer, and the main 
mass is reserved to be transformed by vitellopliags into a 
form which can be absorbed by the archenteron to feed the 
rapidly developing ectoderm, which has now used up it& 
original supply of yolk. 
The inner yolk pyramids persist as such for a considerable 
time, but later, when their yolk granules have been much 
reduced, they seem to disappear. 
Origin of Body Cavities. 
As development proceeds and the anterior part of the 
endoderm increases in size, a space (the archenteron) appears 
between it and the central yolk mass (PI. 13, fig. 19, * arch .). 
Posteriorly the endoderm is still in close contact with the 
yolk mass, but later a few cells, evidently arising from the 
yolk mass, as the endodermal cells did, appear below it on 
the yolk. These increase in numbers and ultimately form a 
distinct layer, so that the endoderm here seems double. The 
cavity between these two layers is very evidently the 
beginning of the first pair of posterior body cavities (PI. 13, 
fig. 19, and PI. 14, fig. 20, b. c. 2 ). The second pair of body 
cavities is subsequently formed in a similar manner (PI. 13, 
fig. 18, and PI. 14, fig. 20, b. c. 3 ). The body cavities may 
therefore be regarded as of endodermal origin, which, though 
not typically enteroccelic, is a modified form of such a method 
of development. At later stages both pairs of body cavities 
may be seen with a complete epithelial lining (PI. 14, fig. 21, 
b. c. 2 and b. c. 3 ). 
The definite origin of the single anterior body cavity was 
not observed until later, but it may be mentioned here that it 
is developed from the anterior part of the archenteron. 
The Laeva. 
The structure of the larva soon after hatching is not very 
different from that of the late embryo, but certain points. 
