THE GENERAL SCHEME 



ishment from the tissues of the mother, but they are not 

 actually attached to her. In some species of dogfish, the lower 

 part of the egg duct is expanded into a special chamber. In 

 this the embryos are retained. The lining of the chamber is 

 thrown into a mass of finger-like projections, between which 

 grow similar projections from the belly wall of each embryo. 

 Nutriment brought to this zone of interlacement by the blood 

 vessels of the mother filters through the coverings of the 

 two sets of projections into the blood vessels of the embryo, as 

 moisture filters into the roots of a tree. Much the same 

 arrangement prevails in the viviparous snakes. 



In mammals the brood chamber is more than a mere dilata- 

 tion of the oviduct. It becomes a special organ, the uterus, 

 which has thick muscular walls, to enable it to withstand the 

 distention produced by large embryos during weeks or months 

 of development, and afterward to expel them when the time 

 comes for their birth. The attachment between mother and 

 child (the placenta^ to be described more fully later) becomes 

 very intimate and very effective in transmitting nutritive 

 substances to the embryo and carrying waste products away. 

 Instead of being thrust into the outside world as an unpro- 

 tected egg, the mammalian infant is sheltered and nourished 

 in the uterus for a long time — three weeks in the mouse, four 

 months in the pig, nine months in man, two years in the 

 elephant. Even after so long a period of gestation, when it 

 enters the world it is still dependent upon the body and secre- 

 tions of the mother, for it cannot do without the milk she 

 provides for its nourishment. 



A modest word about the father may be in order at this 

 point. It will perhaps seem from our sketch of his biological 

 function that in all the various races of animals his duty and 

 usefulness are done when he has put his sperm cells where 

 they can reach an ovum, serving thus to set the mechanism 

 of the egg into action and to contribute his equal share to the 

 heredity of his offspring. The rest takes care of itself in lower 



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