50 EVOLUTION AND DISEASE. 
and undergone gradual 
transformation into a spinal 
cord. These facts were 
based on the embryological 
history of the parts and 
from a study of the malfor- 
mation exhibited, not rarely, 
by this central canal and 
the permanent alimentary 
canal. 
The mode of development 
of the parts may be briefly 
\ described. The alimentary 
canal and spinal cord of all 
vertebrates arise on a com- 
mon plan. At a certain 
stage it consists of a U- 
shaped tube, as in fig. 24, 
each limb ends in a cal-de- 
sac, and the ventral limb is 
connected by a hollow duct 
with the yolk sac. 
By a series of secondary 
changes the anterior end of 
the ventral limb of this tube 
is made to communicate 
with the exterior by way of 
the mouth and pharynx, and 
Fic, 25.—A diagram representing the parts in the adult derived from the U- 
shaped tube of the embryo. S, stomodceum; I, infundibulum; Pt, pituitary 
body; T, thyroid; L, lung; M, vitello-intestinal duct; A, allantois; C, 
coccygeal body ; G, post-anal gut ; N, neurenteric passage. : 
‘ 
