THE DEVELOPMENT OP THE TEMNOCEPHALE.E. 
435 
The stages by which the intestine, with its definite epithe- 
lium and septa, becomes differentiated, have not yet been 
ti’aced. It seems probable, however, that a structure which 
makes its appearance below the endocoele, after the brain and 
excretory sacs have become formed, may be concerned in the 
formation of the primitive endoderm. At this stage (PI. 25, 
fig. 20), a cleft or infolding appears among the cells on the 
floor of the cavity. Later this takes the form of a group of 
cells arranged around a small lumen — the appearance being 
very similar to that presented by the first beginnings of the 
endocoele itself. Since this is behind the brain and excretory 
sacs, and is too far forward to be of the nature of a genital 
primordium, it is permissible to suppose that it represents 
the earliest rudiment of the endodermal system. Its further 
history has not been traced; in fact, it soon disappears 
as such ; but if the above supposition as to its nature be 
correct the cells to which it gives rise must extend round the 
yolk between it and the rest of the developing parts, and 
become converted into the intestinal e]uthelium. 
In the latest stage observed within the egg — a stage with 
fully-developed eyes and abundant pigment, and with the male 
part of the reproductive apparatus far advanced — the intes- 
tine is still entirely devoid of lumen, and consists of a solid 
granular mass with numerous nuclei, which are most abundant 
in the peripheral zone. Slender strands passing inwards into 
this solid mass represent the future dissepiments. In the 
case of T. novm-zealandiae, however, in the latest stages 
from the egg ttie intestine has developed a lumen, and the 
epithelium is recognisable, though the septum still persists 
shutting off the cavity of the pharynx from the exterior. It 
is a remarkable fact, which is probably of significance in 
connection with the function of this persistent septum, that 
in nearly all sections of late stages with fully-developed 
pharynx the lumen of the latter is found to be filled with a 
mass of yolk-granules wliich have evidently become detached 
from the central body of yolk, and have been prevented by 
