OB.LVIII.] HEAD AND TAIL FOLDS. 813 



of blastoderm takes place at the caudal extremity, and thus the 

 head- and tail-folds are formed. 



Similar depressions mark off the embryo laterally, until it is 

 completely surrounded by a sort of moat which it overhangs on 

 all sides, and which clearly defines it from the yolk. It will be 

 understood that in mammals the yolk-sac is comparatively 

 small. 



This moat runs in further and further all round beneath the 

 overhanging 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 yolk, corresponding to the well or undecked portion of the 

 canoe. 



This communication between the embryo and the yolk is gra- 

 dually contracted by the further tucking in of the blastoderm from 

 all sides, till it becomes narrowed down, as by an invisible con- 

 stricting 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 

 visceral plates. 



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

 folding of the visceral plates, just as the neural cavity is pro- 

 duced by the upward growth of the dorsal laminae, the difference 

 being that, in the visceral or ventral laminae, all three layers of 

 the blastoderm are concerned. 



The folding in of the splanchnopleur, lined by hypoblast, 

 pinches off a portion of the yolk-sac, enclosing 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 communicates at first freely, and then by a 

 narrow tube with the yolk-sac. 



The cavity within the hypoblast thus becomes divided into two 

 portions which communicate through the vitelline duct ; the 

 portion within the body gives rise, as above stated, to the diges- 

 tive canal, and that outside the body remains for some time as 

 the umbilical vesicle or yolk-sac (fig. 629, v). The hypoblast 

 forming the epithelium of the intestine is continuous with the 

 lining membrane of the umbilical vesicle, while the visceral plate 

 of the mesoblast is continuous with the outer layer of the umbili- 

 cal vesicle. 



The above details will be clear on reference to the accompany- 

 ing diagrams, some of which, however, allude to structures we 

 have not as yet touched upon. We may here mention three 



