THE PERPETUATION OF LIFE 335 



a few other forms it may be very large. In the egg of 

 a bird the original egg cell forms the part which we call the 

 yolk. This yolk after its discharge from the ovary 

 receives first a coating of albumen, or white, and then the 

 shell during its passage down the oviduct, so that the 

 egg in this case consists of an enormous cell, the yolk, 

 plus the surrounding materials which are secreted by 

 the glands of the oviduct. 



Animals which lay eggs are said to be oviparous, but 

 there are many animals such as all the mammals (with 

 the exception of the monotremes) in which the develop- 

 ment of the embryo takes place in the body of the mother. 

 These forms are called viviparous since they bring forth 

 living young, but in all viviparous animals the embryo 

 arises from an ovum or egg, just as in those forms which 

 lay eggs. When eggs are of large size it is due to the 

 presence of yolk or other material which affords food for 

 the developing embryo. The eggs of the viviparous 

 mammals are very minute. They are always fertilized 

 within the body of the female, usually while in the oviduct, 

 and they undergo development within the uterus. In all 

 mammals higher than the marsupials the embryo becomes 

 attached to the wall of the uterus by an organ containing 

 numerous blood vessels, called the placenta. Through 

 this organ nutriment is carried by means of the maternal 

 blood to the embryo, whose own blood vessels extend into 

 the placenta and there absorb the food material that 

 passes out by osmosis from the blood of the mother. The 

 blood vessels that lead from the embryo to the placenta 

 are contained in the umbilical cord which enters the 

 embryonic body at the point called the navel. The 

 placenta is cast out soon after birth. 



The first clearly marked stage of development that 

 occurs after the fertilization of the egg is the process of 



22 



