206 
COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
In the majority of vertebrates the blastopore closes behind, so that 
Fic. 208.—Reconstruc- 
tion of‘ alimentary canal of 
human embryo, after His. 
al, allantois stalk; cl, cloaca; 
g, glottis; hk, hyoid arch; k, 
liver; /u, lung; md, mx, man- 
dibular and maxillary arches; 
n, nasal pit; 0, omphalomesa- 
raic vein; s, stomach; v, vis- 
ceral arches; vi, vitelline 
stalk; w, Wolffian body. 
the anus is a new formation, although it arises 
in the line of closure. In the amniotes this 
opening is preceded by the formation of a 
pocket, the proctodeum, similar to the 
stomodeum, and opening later into the 
mesenteron in the same way. In the adult it 
is impossible to find any lines separating the 
three regions, stomodeum, mesenteron and 
proctodeum. 
The proctodeum lies wholly behind the entrance 
of the urogenital ducts into the cloaca. The ecto- 
derm of the stomodeum extends inward as far as the 
posterior teeth, following the outline of the jaws. On 
the dorsal side of the oral cavity two pits persist for 
some time, the limits of ectoderm and entoderm pass- 
ing between them. ‘The posterior of these, Seessel’s 
pocket, is of unknown significance. The other, 
Rathke’s pocket (fig. 253), lies just in front of the 
oral plate. It marks the point of invagination of the 
hypophysis (p. 148) and remains open for a time as 
the hypophysial duct (fig. 148). 
In some teleosts, where the stomodeal ingrowth is 
slight, the mouth appears at first as a pair of per- 
forations in the oral plate, these later coalescing to 
form the permanent mouth. This condition lends 
plausibility to the view that the vertebrate’ mouth has 
arisen from the coalescence of a pair of gill clefts. 
Except in the higher mammals the ento- 
dermal part of the alimentary canal contains a 
large amount of food yolk in the early stages. 
In the sauropsida this is so abundant that the 
whole cannot be contained in the body walls, 
and hence it causes ‘the ventral side of the 
canal to protrude as a yolk-sac, which is 
gradually absorbed with the digestion and re- 
moval of the yolk by the blood-vessels. 
The first differentiation in the mesenteron 
is the development of a ventral diverticulum, 
the anlage of the liver, which arises just caudal 
to the head. This divides the alimentary canal into pre- and post- 
