THE DEVELOPMENT OP THE TEMNOCEPHALE.E. 
426 
large cells^ on either side of the rudiment of the brain, and 
somewhat behind it. These cells are situated immediately 
below the thin epithelial lining of the endocoele cavity. One 
cell becomes considerably enlarged, and a narrow sinuous 
channel becomes formed in its substance. This channel 
becomes continued thi-ough a second and a third cell placed 
in close apposition with the first. As subsequent stages sliow, 
these constitute the rudiments of the terminal conti'actile 
sacs and the beginnings of the main vessels of the excretory 
system. 
The pharynx is formed from a number of cells which 
become arranged after the manner of an epithelium imme- 
diately beneatli (i. e. outside of) the thin syncytial epithelium 
of the endoccele on the dorsal side. The time of appearance 
of this layer varies somewhat. Usually it is not seen until 
both the brain and excretory rudiments have become well 
established. The part of the Avail of the cavity on which it 
is situated becomes somewhat rounded off, thougrli still re- 
maining in wide communication with the rest. Posteriorly a 
short prolongation without cellular lining extends for a short 
distance backwards into the mass of yolk; this represents 
the lumen of the intestine (see PI. 24, fig. 11). 
The cavity of the endocoele as a whole decreases much in 
size. The thin layer of yolk by which it is separated from 
the exterior becomes still more attenuated, but still remains 
as a definite septum (Pis. 24 and 25, figs. 11, 12, 13, s.), 
cutting off the whole internal cavity from the exterior. 
^Vhen the brain and the excretory sacs are first formed, 
the blastoderm does not extend anteriorly or posteriorly 
beyond the limits of the endocoele. But a little later it begins 
to grow backwards to form the foundations of the posterior 
parts of the embryo. This baclcAvard extension (PI. 25, fig. 
14) is made up, like the main body of the blastoderm, of cells 
of three sizes, small, intermediate, and large; and these are 
arranged in groups with a marked bilateral symmetry. The 
large cells are formed by a proliferation of the membi’ane 
lining the endocoele. Outlying small cells are to be found 
