206 EMBRYOLOGY. 



existence. The next chapter will explain what becomes of the 

 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 vertebra ted 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 Reptiles 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, tail-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 Fishes 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-like 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 Reptiles and Birds the extra-embryonic region furnishes, 

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

 which complicate the development. 



CHAPTER XI. 



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 



