64 
ANNIE MATTHEWS. 
becoming' smaller and more numerous. Between the periods 
of division the segments are flattened, but just after segmen- 
tation they are very prominent, so that the contour alters 
considerably. The late morula is approximately spherical, 
and though during late segmentation it may become tem- 
porarily oval it soon regains its round shape. The first 
delamination cleavage occurs when the nuclei of the sixteen 
cell stage divide (PI. 3, fig. 11, D. N.). Some of the spindles 
lie along* a radius of the sphere, and hence the resulting* 
daughter nuclei lie similarly, so that a cell is split off towards 
the centre of the embryo in all such cases, and thus an inner 
endodermic layer arises (PI. 3, fig. 12, End.). Other spindles 
lie in a plane at right angles to the radius, and hence these' 
daughter cells lie side by side with the parent cell in the 
outer ectodermic row (PI. 3, fig. 12, N.). It is thus evident 
that all of the sixteen cells do not simultaneously contribute- 
to the endoderm layer, and while both ectoderm and endoderm 
cells continue to divide, later radial spindles in the ectoderm 
afford evidence that the early endoderm cells are continually 
reinforced from the ectoderm (PI. 3, fig. 12 ) } Hence from 
the thirty-two celled stage onwards the larva is two-layered 
(Text-fig. 34). The outer layer is somewhat irregular at first 
(PI. 3, fig. 12), and, as Wilson says of Renilla ( 16 ), the 
ectoderm cells dovetail into those forming the inner mass. 
The endodermic yolk globules are much larger than those of 
the ectoderm at this stage (PL 3, fig. 21). As the number 
of cells increases the ectoderm becomes a more regular row 
of cuboid cells, staining much more deeply than the inner 
layer of larger polygonal endoderm cells (PI. 3, fig. 21). 
Towards the end of the morula stage about thirty-two small 
cells can be counted round the circumference of the sphere, 
the ectoderm being now columnar (Text-fig. 34). The yolk 
in the endoderm has been partly used up, the vesicles having 
become smaller and similar to those in the ectoderm (PI. 3, 
fig. 20). 
1 The writer hopes to discuss the question of the origin of th£ 
endoderm in greater detail in a subsequent paper. 
