698 GENERAL THENOMEXA OF E:^rERYOXIC DEYELOPilEXT. 



suiTonnding the A'ascular area and receiving the blood from the capillary 

 or subdivided vessels of the area within. 



Alimentary Canal. — The formation of the rudiment of the ali- 

 mentary canal or pi'imitive intestine takes place below or within the 

 boat-shaped part of the embryo previously described by the folding in, 

 soonest at the cephalic and later at the caudal extremities, and subse- 

 quently along the sides, first of the hypoblast, from which the epithelial 

 lining only of the intestine is formed, and afterwards of certain parts of 

 the splanchnoplenre section of the mesoblast which give rise by their 

 meeting in the middle to the mesentery, and furnish in their extension 

 over the intestinal tube the muscular and peritoneal coats and the 

 connective-tissue and vascular elements of the gut. 



The primitive alimentary canal is thus constituted in its early form 

 by an anterior and posterior ca3cal tube, of which the anterior is the 

 first produced, — both of them closed at the extremity by the reflected • 

 layers of the blastoderm, — and by a wide middle part between the tubular 

 portions, which at first has the form of a groove or gutter running under 

 the vertebral axis of the embryo, and completely open below into the 

 cavity of the yolk-sac. As development proceeds, the intestinal folds 

 involve m.ore and more of this central open part and convert it into 

 the tubular form ; and the opening into the yolk is thus gradually 

 narrowed, while the reflected part of the blastodermic layers which 

 pass between the yolk-sac and the intestine becomes lengthened out so 



y i,^s ^-V^' 



Pig. 510. — Diagrammatic Sectiox through the Ovum of a Mammal in the loxg 

 AXIS OP THE Embryo. 



e, the cranio-vertebral axis ; /, /, the cepbalic and cauflal portions of the primitive 

 alimentary canal ; a, the amnion ; a', the point of reflection into the false amnion ; v, 

 yolk sac, communicating with the middle part of the intestine by v i, the vitello-intestinai 

 duct ; M, the allantois. The ovum is surrounded externally by the villous chorion. 



