ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY. 205 



intestinal yolk-sac and dermal yolk-sac. The former is simply a. 

 hernia-like evagmation of the intestinal canal, and, like it, is 

 composed of three layers : 



(1) The intestine-glandular layer (ik), the entoblast or secondary 

 entoderm, which encloses the yolk ; 



(2) The visceral middle layer, or the pleuroperitoneal epithelium 

 (mjfc 2 ) ; and 



(3) The intermediate layer (Zwischenblatt), in which have been 

 developed the vitelline blood-vessels, which at the beginning of the 

 circulation of the blood have to conduct the liquefied nutritive 

 material from the yolk-sac to the places of embryonic growth. 



The dermal yolk-sac is, as a continuation of the body-wall, likewise- 

 composed of three layers the epidermis (a% the parietal middle 

 layer (mk l ), and the connective-tissue intermediate substance 

 (Zwischensubstanz). 



It has already been stated that the constricting-off of the yolk-sac 

 from the embryonal body is quite variable in extent, and can go so- 

 far that the connection between the two is kept up only by means 

 of a narrow stalk. A more careful examination shows that in the 

 latter case the stalk itself is composed of two narrow tubes one 

 within the other (Plate I., fig. 7), of which the outer unites the 

 dermal yolk-sac (hs) to the ventral wall of the body, and the inner 

 the intestinal yolk-sac to the intestinal canal. The former is called 

 the dermal stalk, the latter the intestinal stalk (dn) or vitelline 

 duct, ductus vitello-intestinalis. The place of attachment of the 

 dermal stalk in the middle of the ventral surface of the embryo i& 

 called the dermal navel (hn) ; the corresponding place of attachment 

 of the intestinal stalk to the wall of the intestine the intestinal 

 navel (dn). The embryonic body-cavity opens out between the two, 

 and is continuous with the fissure between dermal and intestinal 

 yolk-sac with the " extra-embryonic body-cavity " or the blasto- 

 spheric ccelom (lh 2 ). 



The ultimate fate of the yolk-sac in the Fishes is the same as in 

 the Amphibia. It is still employed, even in the extreme case of the 

 Selachians, for the formation of the wall of the intestine and that 

 of the body. The more its contents are liquefied and absorbed, 

 the more the yolk-sac shrivels. When the intestinal yolk-sac has 

 become very small, it is drawn into the body-cavity and finally 

 serves to close the intestinal navel, just as the dermal yolk-sac upon 

 its disappearance closes up the dermal navel. With the lower 

 Vertebrates a shedding of the embryonic parts has not yet come into* 



