Growing Period of Various Mammals 



largest species, the kangaroos, when only about two inches long. The mother 

 places the newborn young in the brood-pouch, where they are kept protected 

 and warm and where they feed on milk from the glands of the mother. 



As we go from the lower orders of mammals to the primates, we find that 

 the young are protected and nourished for a longer period preceding birth. 

 And the young depend upon their parents for longer and longer periods after 

 birth also. The table above compares several species of mammals, including 

 man, in regard to the period of gestation within the mother, the time to sexual 

 maturity, and the total length of life. 



The Embryo in Mammals Among all except the pouched and the egg- 

 laying mammals (see Appendix A), the embryo remains within the uterus of 

 the mother until it attains a body form resembling in a general way that of the 

 adults of the species. While still a tiny spherical mass, suggesting a golfball 

 with a fluid interior, the fetus attaches itself to the lining of the uterus (see 

 illustration, p. 383). Outgrowths from the surface cells dig into the lining 

 by a sort of digestive process, with the result that the fetus comes to be sur- 

 rounded by lymph from the mother's capillaries. Nourished through the 

 extensions into the tissues of the uterus, the inner cells of the fetus grow 

 and divide rapidly, and the mass takes on a definite form — which is steadily 

 changing (see pages 354-356). 



The outer layer, with its extensions, also enlarges and acts as a special 

 nutritional organ until the embryo completes its development. This special 

 organ is called the placenta and has multitudes of villi, or outgrowths, 

 which contain blood vessels. These villi are somewhat like the villi of the 



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