206 . EMBRTOLOQY. 



existence. The next chapter will explain what becomes of tho 

 yolk-sac in the case of Reptiles and Birds. 



Summary. 



1. In the case of Vertebrates whose eggs contain little yolk, the 

 embryo after the development of the germ-layers takes on an 

 elongated, fish-like form. 



2. In eggs with abundant yolk, the body of the vertebrated animal 

 is produced by only a small region of the germ-layers (the embryonic 

 fundament) ; the far greater extra-embryonic area is employed for 

 the formation of a yolk-sac and of embryonic membranes (the latter 

 only in Eeptiles and Birds). 



3. The separate layers of the embryonic fundament constrict them- 

 selves off from the extra-embryonic territory, and at the same time 

 become folded into tubes — the somatopleure into the tubular body- 

 wall, the splanchnopleure into the intestinal tube (head-fold, taU-fold, 

 lateral folds, intestinal groove, intestinal fold). 



4. The extra-embryonic territory of the germ-layers remains in 

 ■continuity with the two tubes by means of a stalk-like connection. 



5. In Kshes the extra-embryonic territory of the germ-layers 

 becomes the yolk-sac, which is composed of two sacs, the intestinal 

 and the dermal yolk-sacs, separated from each other by a pro- 

 longation of the embryonal body-cavity. 



6. The place where the dermal yolk-sac is attached to the belly- 

 -wall of the embryo by a stalk-Hke prolongation is called the dermal 

 navel or umbilicus ; the corresponding place of attachment of the 

 intestinal yolk-sac to the middle of the intestinal canal is the 

 intestinal navel or umbilicus. 



7. In Fishes the yolk-sac after resorption of the yolk-material, 

 ■accompanied by the phenomena of shrivelling, is employed for the 

 closure of the intestinal and dermal navels. 



8. In BeptUes and Birds the extra-embryonic region furnishes, 

 in addition to the yolk-sac, several other embryonic membranes, 

 ^which complicate the development. 



CHAPTER XL 



THE FCETAL MEMBRANES OF REPTILES AND BIRDS. 



-As has already been stated, the course of development in all animals 



which do not deposit their eggs in water — in Reptiles, Birds, and 



Mammals — is unusually complicated, owing to the appearance of 



