580 JAMES F. GEMMILL 
central cavity filled with coagulable fluid makes its appearance. 
The egg-membrane is not separated off as a membrane of 
fertilization, but is found to follow closely every surface 
change of contour so long as it is recognizable. As segmentation 
proceeds, non-nucleated portions separate off from the inner ends 
of the cells, and, mixing with the blastocoelic fluid, form a central 
diffuse trophenchyme. At this stage one or two whole cells 
may share the same fate by migrating or getting pushed 
inwards from the surface. Their nuclei proliferate; but, 
soon losing control over the cell-contents which become 
trophenchymal, are destined to degenerate along with the 
other trophenchymal nuclei to be described later. 
A little later the Bolocera egg shows very markedly those 
peculiar surface grooves and foldings which Masterman 
first described in the case of Cribrella (17, p. 8), and which have 
since been noted in many ova (8, p. 12). During this process there 
is a tendency, better marked in some instances than in others, 
for the egg to assume the form of a flattened dise the edges of 
which become turned upwards like those of a saucer. The 
surface grooves and the saucer cavity gradually fill out, so 
that the egg becomes almost spherical again. The saucer 
cavity is accordingly not the archenteron, though gastrulation, 
which soon supervenes, affects the part of the egg-wall 
which was formerly the hollow of the saucer. In the fully- 
formed blastula this part often remains flat while the rest of 
the blastula wall is spherical. 
An important point to note is that as the surface folds smooth 
out, many single cells and groups of cells are nipped off from 
the recesses, and migrating inwards become included within 
the trophenchyme. I thought at first that these cells were 
going to form the endoderm of the larva. But this is not so. 
Their cell outlines will disappear and their nuclei degenerate. 
Gastrulation.—In typical cases (see e.g. figs. 7-9) 
a relatively large portion of the blastula wall shows 
flattening and sinks gradually downwards, the margins of 
this portion closing in slowly to form the lp of the blasto- 
poric opening. At the same time this lip becomes slightly 
