ALIMENTARY CANAL IN LEPIDOSIEEN AND PROTOPTERUS. 487 
gut into the cavity of the mid-gut persists throughout life, 
and forms the characteristic “ pyloric valve.” 
The main features in the topographical evolution of the 
mid-gut and hind-gut will be gathered from an inspection of 
the figures of the external features (see Keibehs “Nor- 
mentafeln,” Heft x) together with text-figs. 3 A — d, 4 A — d, 
and 5. 
In correlation with the function of this part of the gut 
as the storehouse of food material on which the young 
animal has to subsist for a prolonged period, its cells remain 
for long laden with yolk, and its developmental progress is 
correspondingly retarded. The first conspicuous change 
consists in the rapid elongation of the hinder part of the gut 
which accompanies the rapid growth of the hinder trunk 
region (see figures of external features, stages XXV — XXX). 
The anterior region of the mid-gut for a considerable time 
retains its spheroidal shape, and it is this which gives the 
characteristic tadpole-like appearance to these stages. About 
stage XXXIII in Lepidosiren, but not till about stage 
XXX V in Pro top ter us, the prominent bulging of the ante- 
rior part of the mid-gut disappears, and the tadpole-like 
appearance is finally lost. 
The general appearance of the mid- and hind-gut as seen in 
a dissection of a young Lepidosiren of stage XXXII is shown 
in text-figs 3 a and 4 a. The prominent rounded bulging 
of the anterior end is already at this stage giving place to a 
gradual tapering. Towards the hinder end a faint spiral 
marking caused by a shallow groove traversing the surface 
of the gut rudiment foreshadows the development of the 
spiral valve. 
In the mid-dorsal line a broad valley passes back for some 
distance, and in this lies the rudiment of the lungs. Text- 
fig. 4 A shows how the fore-gut in the I’egion of the glottis 
bends abruptly towards the left side, passing into the mid- 
gut far to the left of the mesial plane. 
In the dissection of stage XXXV (text-figs. 3 b, 4 b and 5) it 
is seen that the swollen mass of yolk in front has been greatly 
