EXCRETORY AND REPRODUCTIVE SYSTEMS 353 



a helpless condition. In some species, this food is freshly- 

 caught, in others it is regurgitated, while in the pigeon the 

 lining of the crop comes away forming a mucous secretion 

 known as " pigeon's milk." 



In all mammals, the young are fed on milk produced by the 

 mammary glands of the mother, and they are born alive in all 

 except the Monotr ernes, in which they are hatched from eggs. 

 The mammary glands of the Monotremes are primitive in that 

 they do not have proper teats, but merely exude the milk 

 which is lapped up by the young. In the Monotremes and 

 Marsupials, the ventral surface of the belly of the female is 

 modified to form a pouch, or marsupium. In the Monotreme, 

 the egg is hatched in the pouch. The young Marsupial is born 

 very early after a short period of gestation in the uterus, and 

 in a very undeveloped condition. It becomes attached to a 

 teat of the mammary glands which are situated in the pouch. 

 There it completes its development. In the higher mammals, 

 or Placentals, the allantoic placenta is well developed, and the 

 period of gestation is long. During this time the embryo is 

 able to develop to a high degree of perfection, such as would 

 not be possible without a lasting physiological connexion with 

 the mother in the form of a placenta. After birth the young 

 is supplied with milk by the mother until it is weaned and 

 able to feed for itself. Another feature of the mammals is 

 that they go through a period of " childhood," during which 

 they play, fed by the mother and protected by her and the 

 father. Parental instincts reach their highest form in man, 

 whose superiority in mind and body is conditioned by the 

 relatively very great length of time spent in development, 

 both before and after birth. 



There may be one or several Embryos developing in the 

 uterus at the same time. Each embryo develops from a 

 separate egg except in cases of true twinning ; here, as in the 

 armadillo and possibly in man, the twins arise by a process 

 of fission of one blastocyst which has developed from one 

 egg- 



2 A 



