THE DEVELOPMENT OF THE TEMNOCEPHALE.E. 427 
sions the layer of yolk, which separates it ou the ventral side 
from the exterior, becomes greatly reduced in thickness. At 
the same time, subsequently to the formation of the embry- 
onic brain and excretory sacs, a portion of its wall just behind 
the brain undergoes modification, a number of large cells 
becoming arranged in an epithelium-like manner beneath the 
thin lining membrane. The cavity then becomes much re- 
duced in size, and the part with the large cells becomes 
rounded off to form the pharynx. Ventrally it continues to 
be separated from the exterior by the original roof of the 
cavity, which becomes reduced to an extremely thin mem- 
brane. Posteriorly a further remnant of the original cavity 
is represented by a very short passage which ends blindly in 
the yolk. The pharyngeal sac and mouth become formed by 
growth of the integumentary and muscular layers around the 
thin membrane that represents the original roof of the cavity; 
but the membrane remains as a distinct, though very delicate, 
partition between the buccal cavity and the pharynx as long 
as the young animal remains within the egg. 
The intestine remains without lumen in the most advanced 
stage observed within the egg, a stage in which all the other 
parts, even the male reproductive apparatus, have reached an 
advanced stage of development. It would, perhaps, be more 
correct to say that the young animal has no intestine at this 
stage, the site of the future intestine being occupied by a 
solid mass of yolk still containing remains of the original 
nuclei of the yolk-cells. 
VI. Development op the Excretory System. 
The excretory system of the adult Temnocephala, 
though constructed on the same type as that of the Platodes 
in general, possesses certain features which, so far as our 
present knowledge extends, are peculiarly its own. The 
presence of contractile terminal sacs through which the 
system communicates with the exterior is not peculiar to 
