156 Thomas Henry Huxley 



detail by a set of investigators. The dog, like all verte- 

 brate animals, begins its existence as an egg ; and this 

 body is just as much an egg as that of a fowl, although, 

 in the case of the dog, there is not the accumulation of 

 nutritive material which bloats the egg of the hen into 

 its enormous size. Since Huxley wrote, it has been 

 shewn clearly that among the mammalian animals there 

 has been a gradual reduction in the size of the egg. 

 The ancestors of the manmials laid large eggs, like 

 those of birds or reptiles ; and there still exist two 

 strange mammalian creatures, the Ornithorhynchus 

 and Echidna of Australia, which lay large, reptilian-like 

 eggs. The ancestors of most living mammalia acquired 

 the habit of retaining the eggs within the body until 

 they were hatched ; and, as a result of this, certain 

 structures which grow out from the embryo while it is 

 still within the egg and become applied to the inner 

 wall of the porous shell for the purpose of obtaining air, 

 got their supply of oxygen, not from the outer air, but 

 from the blood-vessels of the maternal tissues. "When 

 this connection (called the placenta) between embryo 

 and mother through the egg-shell became more perfect, 

 not onl\- oxygen but food-material was obtained from 

 the blood-vessels of the mother; and, in consequence, it 

 became unnecessary for the eggs to be provided with a 

 large suppl}' of food-yolk. Among existing marsupial 

 animals, which, on the whole, represent a lower type of 

 mammalian structure than ordinary mammals, there is 

 more food-yolk than in ordinary mammals, and less food- 

 3-olk than in the two egg-laying mammals. In the 

 ordinary mammals, such as the rabbit, dog, monke}', 

 and man, there is practically no j^olk whatever deposited 

 in the egg ; the egg is of minute size, and the embryo 

 obtains most of its food from the maternal blood. 



