32 GENERAL EMBRYOLOGY. 
a fold, by which the gut is suspended from the under surface of the primitive 
vertebral column; this fold is the mesentery. 
The diaphragm, which is formed at a later period and which separates the 
thorax from the abdomen, divides the ccelom into pleural and peritoneal portions. 
The primitive alimentary canal is almost a straight tube, blind at both its 
extremities, and communicating only with the cavity of the yolk sac. As yet there 
is no mouth and no anal passage or aperture. The simple tubular canal is divisible 
into fore-gut, mid-gut, and hind-gut, parts which are conveniently associated 
developmentally with definite portions of the fully-formed alimentary canal. 
Thus the fore-gut is converted into the pharynx, cesophagus, stomach, and the 
greater part of the duodenum; whilst from the mid-gut and the hind-gut the rest 
of the small intestine (je- 
junum and ileum), and the 
whole length of the large 
intestine (czecum, colon, and 
Forebrain rectum), are formed. There 
‘Optic vesicle 18 no sharp limit between 
Bucco- the mid-gut and the hind- 
pharyngeal out, or between the portions - 
of the intestinal canal which 
- Mandibular 
arch develop from them. 
Diverticular outgrowths 
Otic vesicle 
Pharynx —_f 
Esophagus —f 
é atheseon from the entoderm of the 
ss bronchi \ - Heart primitive alimentary canal 
see form the rudiments of the 
intestinal glands, including 
Eiver the liver and pancreas, of the 
Duodenum : 
respiratory apparatus, and 
of the thyroid and thymus 
glands. Details of the for- 
mation of these structures 
are given in the special de- 
scription of the development 
of the system to which each 
belongs. 
Except with respect to the 
anterior part of the fore-gut, 
the changes in shape and 
position which the originally 
*F simple alimentary tube un- 
Cloacal membrane —_— Body stalk dergoes during its conversion 
Fic. 26.—DIAGRAM REPRESENTING THE CoNDITION or THE Az Ito its final or adult form 
MENTARY CANAL IN A HumaN Empryo apour FirreeN Days are described in the account 
ieee peice irom pels): of the development of the 
The visceral clefts are formed, and the subdivisions of the fore-gut, digestive organs ; but the 
together with the rudiments of the bronchi and liver, are distinct. 5 ° 
development of the pharynx 
and the structures associated with it, and the formation of the mouth and anus, 
may be considered now. 
Development of the Pharynx and Stomatodeum.—The development of the 
anterior part of the fore-gut into the pharynx and the floor of the mouth 1s so inti- 
mately associated with the formation of a primitive mouth, the stomatodeeum, that 
the two must to a certain extent be considered simultaneously. 
The stomatodeum first appears as a depression between the head and the peri- 
cardial region. It is produced by the downward growth of the forepart of the 
head in front and the bulging forward of the pericardium behind, and it is separated 
from the anterior end of the fore-gut by the bi-laminar bucco-pharyngeal membrane. 
When the stomatodeeum first appears it is not enclosed laterally; but at a later 
period side boundaries are formed, and the space is developed into the upper part 
of the mouth and the nasal cavities. 
Yolk-sae 
Mid-gut 
‘ V7 ; ee 4 
Hind-gut 
