276 



ELEMENTS OF BIOLOGY 



protoplasms arise during the developmental process. It is therefore 

 plain that the newly forming animal must have at its disposal an 

 abundance of food to provide both energy and materials for devel- 

 opment. All types of eggs that are fertilized outside the body of the 

 female obviously develop independent of the maternal body. Of 

 eggs that are fertilized within the female, some are expelled before 

 development proceeds very far, and some spend the entire period of 

 development within the maternal body. The former type is de- 

 scribed as OVIPAROUS and the latter as viviparous animals. In ovi- 



CHtCK EMBRYO 



NUTRITIVE AND 

 RESPIRATORY 

 ORGAN 



YOLK SAC 



Fig. 188. — An embryo chick with its yolk sac, showing the circulation of the blood 



over the yolk area. (After Duval.) 



parous forms that do not develop into free swimming or feeding 

 larvx, for example the chick, the energy and materials required for 

 the processes of development are, except for oxygen, contained in 

 the tgg as yolk and other materials. Embryos in such eggs as the 

 chick develop a sac which encloses the yolk; blood vessels and 

 blood cells develop in the wall of the yolk sac and transport the 

 food materials into the embryonic body and carry outward to the 

 surface of the yolk the gaseous wastes of metabolism (Fig. 188). 

 The wall of the yolk sac thus serves as the chief respiratory and 

 nutritive organ during much of the period of development. 



The Placenta. Some viviparous animals merely incubate the 

 eggs in the maternal body during development, the food supply 



