THE PRIMITIVE ALIMENTARY CANAL. 31 
THE DEVELOPMENT OF THE PRIMITIVE ALIMENTARY CANAL. 
When the cephalic, caudal, and lateral folds are established, and the general 
outline of the embryo is clearly defined, its dorsal and lateral surfaces and its 
anterior and posterior extremities are easily recognisable, and, as the embryo is 
folded off from the surface of the blastodermic vesicle, a portion of the blastodermic 
cavity is enclosed within it; this is the primitive alimentary canal. It is simply 
an incomplete tubular cavity, situated beneath the notochord, which is bounded in 
front by the head fold, behind by the tail fold, and laterally by the lateral folds, 
but is widely open below and continuous with the cav ity of the yolk sae. 
As the head of the embryo grows more rapidly than any other part, the head 
fold is more marked than the other folds, and with its formation the pericardial 
area is bent round until it becomes ventral in position, its original upper and lower 
surfaces being reversed (Fig. 25). It is owing to this change of relative position 
Spinal cord Notochord 
| 
Rhomboidal sinus 
Mid-brain per 
Primitive streak 
Cloacal membrane 
Ne Placental area 
Amniotic fold 
Placental 7 
Alimentary cana] 
, -Pericardium 
Yolk sae 
- EC 
SpM, SoM 
Bueco-pharyngeal membrane 
Fic. 25.—DIaGRAM OF A DEVELOPING OVUM, SEEN IN LONGITUDINAL SECTION. 
The folding off of the embryo has commenced, and the downward bend of the head fold in front has invagi- 
nated the amniotic area. The tail fold is partly formed, and the primitive alimentary canal, closed in 
front by the bucco-pharyngeal membrane and behind by the cloacal membrane, is distinguishable ; it 
= communicates freely with the yolk sac by a wide umbilical aperture. 
C. Cclom. EN. Entoderm. SoM. Somatic mesoderm. 
EC. Ectoderm. M. Mesoderm. SpM. Splanchnic mesoderm. 
that the ventral wall of the alimentary canal is completed in front, and it is 
obvious that its anterior limit corresponds to the bueco-pharyngeal area of the 
blastoderm. The part so closed in constitutes the fore-gut. 
The tail fold at this period is small, but it limits the primitive gut behind. 
The ventral closing of the posterior end of the primitive alimentary canal to 
form a hind-gut is produced, as in the case of the fore-gut, by bending of the 
embryonic area. This takes place in the region of the tail ‘fold : but the posterior 
part of the embryonic area retains for a considerable time its original position, and 
forms a connecting stalk, termed the body-staik, between the embryo and the 
chorionic area of the blastoderm. Ultimately, however, this terminal section of the 
embryonic area is reversed in position, its posterior end being carried forwards till 
it forms the posterior boundary of the umbilical orifice, and the ventral wall of the 
hind-gut is thus completed. 
The rest of the primitive alimentary canal constitutes the mid-gut. It remains 
for some time in free communication with the cavity of the yolk-sac, and this com- 
munication between the alimentary canal and the yolk-sac at a later stage forms a 
tubular passage, the vitello-intestinal duct. 
The entoderm forms the lining epithelium of the alimentary canal, but this is 
invested by the splanchnic layer of the mesoderm, and it is ‘separated from the 
somatopleure or body wall by the ccelom or body cavity. As the splanchnic 
mesoderm passes on each side to its continuity with the somatic mesoderm 1t forms 
