Development of the Mammal 285 



on the other hand, there is practically no food 

 yolk, so that almost from the beginning, the 

 embryo is dependent upon a source outside of 

 itself for food ; this source is mainly the blood 

 of the mother, and to make it available there 

 is developed what is known as the placenta, 

 to be described later. 



PL 



Sn. 



YIG. 85. FULL-GROWN HUMAN OVUM BEFORE 

 MATURATION. (From Minot, after Nagel.) 



cor. r.^ part of corona radiata. Z, zona pellucida. 

 /V., protoplasm. K, yolk. A'w., nucleus. 



THE SEX CELLS 



It will be well to begin the discussion of the 

 embryology of the mammal with a description 



