166 
J. BRONTE GATENBY. 
Fig. 24 of PL 11a transverse section of the embryo is drawn 
at a stage when the cell outlines begin to appear. At the 
places where the body cavity is formed, the syncytium becomes 
thin and vacuolated, and between the future cell elements, 
cell walls are deposited. In PI. 11, fig. 24, the large endo- 
derm cells have become arranged in a definite manner 
(END.N .) , and the beginning of the lumen of the future gut 
is seen at GL. Beneath the ring of endoderm nuclei (END.N.) 
a large cavity ( CAV .) has already appeared, but otherwise 
the separation into regions is still slight. On what is the 
ventral side of the embryo, at the letters NCN, will be noticed 
three rows of nuclei. The upper row ( MCN .) just beneath 
the embryonic body cavity (CAV.) becomes detached by 
further vacuolisations in the region marked X, X, and in the 
larva becomes loose in the body cavity (PL 12, fig. 37, MCN.). 
Of the two lower rows, the bottom one, and at least some 
of the upper row nuclei, form the nerve chain of the adult. 
Fig. 39 of PL 12 should be compared with this figure. 
In PI. 12, fig. 39, the body cavity is better formed (CAV.). 
It will be noticed in PL 11, fig. 18, that there are four large 
nuclei marked Z which do not seem to be included in the 
forming gut. The upper two may form such large glandular 
cells as those marked Z in PL 12, fig. 39, for in this figure 
it will be noticed that in places the wall of the gut (GL.) is 
formed of two rows of cells. One can often find very large 
unattached cells in the newly-formed body-cavity, and these 
may break up later on (PL 12, fig. 39, XX.). Immediately 
after the final sorting up of the cell elements, and after each 
nucleus has taken its place, there is an expulsion of super- 
fluous cells, which degenerate either in the hsemocoel, or are 
cast from the surface of the ectoderm (PL 12, fig. 39, X, X.). 
The Formation of Stomodeum, Mesenteron, and 
Proctodeum. 
As far as one can tell in a case where such wide variation 
occurs, the large dorsal mass of nuclei which sinks inwards 
