668 HANDBOOK OF PHYSIOLOGY. 



pletely surrounded by a sort of moat which it overhangs on all sides, and 

 which clearly defines it from the yelk. 



This moat runs in further and further all round beneath the over- 

 hanging embryo, till the latter comes to resemble a canoe turned upside- 

 down, the ends and middle being, as it were, decked in by the folding 

 or tucking in of the blastoderm, while on the ventral surface there is 

 still a large communication with the yelk, corresponding to the well or 

 undecked portion of the canoe. 



This communication between the embryo and the yelk is gradually 

 contracted by the further tucking in of the blastoderm from all sides, 

 till it becomes narrowed down, as by an invisible constricting band, to a 

 mere pedicle which passes out of the body of the embryo at the point of 

 the future umbilicus. 



The downwardly folded portions of blastoderm are termed the vis- 

 ceral plates. 



Thus we see that the body-cavity is formed by the downward folding 

 of the visceral plates, just as the neural cavity is produced by the up- 

 ward growth of the dorsal laminas, the difference being that, in the 

 visceral or ventral laminae, all three layers of the blastoderm are con- 

 cerned* 



The folding in of the splanchnopleure, lined by hypoblast, pinches 

 off, as it were, a portion of the yelk-sac, inclosing it in the body-cavity. 

 This forms the rudiment of the alimentary canal, which at this period 

 ends blindly towards the head and tail, while in the centre it communi- 

 cates freely with the cavity of the yelk-sac through the canal termed 

 vitelline or omphalo-mesenteric duct. 



The yelk-sac thus becomes divided into two portions which communi- 

 cate through the vitelline duct, that portion within the body giving rise, 

 as above stated, to the digestive canal, and that outside the body remain- 

 ing for some time as the umbilical vesicle (Fig. 453, ys). The hypoblast 

 forming the epithelium of the intestine is of course continuous with the 

 lining membrane of the umbilical vesicle, while the visceral plate of the 

 mesoblast is continuous with the outer layer of the umbilical vesicle. 



All the above details will be clear on reference to the accompanying 

 diagrams. 



At the posterior end of the embryo chick, when the amniotic fold is 

 commencing to be formed, and the hind fold of the splanchnopleure has 

 commenced, there remains for a time a communication between the 

 neural canal and the hind gut, which is called the neurenteric canal. 

 It passes in at the point where the notochord falls into the primitive 

 streak. The anterior part of the primitive streak becomes the tail swell- 

 ing, the posterior part atrophies, and the corresponding lateral part of 

 the blastoderm forms part of the body-wall of the embryo. The ante- 

 rior part of the medullary canal having been completely roofed in; the 



