REPRODUCTION 97 



hatched young with a milky secretion which is exuded 

 from the skin of her body and which the young lap 

 up. Another step is taken by the spiny ant-eater, 

 which with its beak-like muzzle thrusts the eggs, as 

 soon as they are laid, into a pouch on the belly similar 

 to that of the kangaroo. The eggs are kept here 

 until they are hatched. The young feed in the same 

 manner on a milky exudation from the skin. 



A further advance is made by the marsupials, to 

 which the kangaroo belongs. Here the fertilized 

 egg (which has no shell) remains in the womb of the 

 mother to undergo the first stages of development. 

 The young are born alive, but in a very immature 

 condition, and have therefore to be carried about in 

 the warm pouch of the mother. This pouch, situated 

 on the belly of the mother, is furnished with milk 

 glands which do not merely open to the skin, but end 

 in proper teats. The newly born kangaroo (hardly 

 one inch long) is attached to the teat of the mother 

 animal, who may be more than man's size. 



As in the marsupials, so we find that in all mammals 

 the fertilized ovum remains in the uterus during the 

 embryonic development. It has no shell, but attaches 

 itself to the wall of the womb and draws nourishment 

 from the internal surface of the womb. As soon as 

 the embryo reaches maturity, birth takes place, the 

 young being expelled through the vagina by the con- 

 tractions of the uterus into the outer world. The 

 period of intra-uterine development is called gesta- 

 tion, and varies in length with different animals, the 

 higher generally having a longer gestation period 

 than the lower (though this is not invariably so), 

 because the more advanced the animal, the longer 



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