EST.\r.u-iiM! vi Off mi: i.\ii:i;vM. POBM Of PHI BODY, - (1 - r > 



intestinal yolk-oac and dermal yolk sac. Tin- former i- -imply a 



In rnia-like evairinat ion of the intestinal canal, and, like r 



composed of three layen : 



(1) 'I'll.- int.stino-L'landular l:i , -the ento --condary 



entoderm, which encloses the yolk ; 



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



(//</.-): and 



(3) The intermediate layer (/wi>rhenblatt ), in which have t)een 

 developed the vitelline blood-vess-ls, which at the berinnin.L' <>f th. 

 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/r), the parietal middle 

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

 (ZuischeiiMibstanz). 



It lias already been stated that the const rictin^-oll' of the yolk 

 from the embryonal body is quite variable in ex tnit, and can ^o so 

 far that the connection between the two is kept up only by mean- 

 of a naiTOw stalk. A more careful examination -hows 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 (/is) to the ventral wall of the body, and the inner 

 the int< stinal yolk-sac to the intestinal canal. The former i- called 

 the dermal stalk, the latter the intestinal stalk ('in] or vitelline 

 duct, diu-tus vitello-inte>tinalis. 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 plaiv of attachment 

 of the intestinal stalk to the wall of the inte.-tine the intestinal 

 na\el (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-einbrvoiiic ln>d\ -cavit v " or the blasto- 

 s])hei-ic coelom (///-). 



'I'he ultimate fate of the yolk-sac in the Fishes is the -ame as in 

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

 Selachian-, for the formation of the wall of the intestine and that 

 of the body. The more its content- are liquefied and al>>..rl> . d, 

 the more the yolk-sac -liriv. 1-. When the int* stinal yolk-sac has 

 become veiy >maii. it is drawn into the body* vd finally 



B6TV< - to close the intestinal navel, jn-t u the dermal yolk-sac upon 

 it- disappearance closes up the dermal na\'el. With the lower 

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



