DEVELOPMENT OF THE HOUSE. 19 



problems which have long been puzzling them, and sometimes 

 urgently pressing for solution, there is no apology needed for 

 having treated this subject on strictly scientific lines. 



Summary. 



I have endeavoured to show that the horse embryo, like the 

 young opossum, is inclosed by a special inner tunic (the amnion), 

 and provided with a pair of appendages (the yolk sac and the 

 allantois), and, further, that a thin outer cellular tunic (the 

 embryonic sac) completely envelops the embryo and all its 

 belongings. The fluid which fills up the space between the 

 embryo and the amnion forms a sort of water-jacket, which is 

 doubtless most useful alike for the protection of the embryo and 

 its dam. The yolk sac, unlike the corresponding structure in the 

 chick, is empty, for the simple reason that there is plenty 

 nourishment available in the uterus or in the blood abundantly 

 coursing through its walls. The more readily to secure this 

 nourishment, part of the yolk sac fuses with the outer cellular 

 tunic to form a sort of filter, through which the nutriment enters 

 the originally empty sac. The nutritive material, after it may 

 be undergoing some chemical change, finds its way from the 

 cavity of the yolk sac into the blood-vessels which ramify in its 

 wall as far as the edge of the filter or absorbing area. 



In the opossum, and the vast majority of the marsupials, the 

 young are born in a very immature condition — as soon as they 

 are able to seize and hang on to the teats. These ancient forms 

 have not discovered that, by utilising the allantois (originally a 

 breathing organ), they might considerably prolong the protection 

 afforded by the uterus ; l that when the yolk sac fails, by throwing 

 out root-like processes from the allantois, they might tap an 

 almost inexhaustible food supply. But all the higher forms (the 

 Eutheria) utilise the allantois, and some of them are already so 

 perfect at birth that the time may come when the milk glands 

 (very old-fashioned organs), so essential to the marsupials, may 

 in some cases be entirely, or almost entirely, dispensed with. 



1 The Bandicoot is the only marsupial in which the allantois has been shown 

 to take part in nourishing the embryo. 



