SEC. 2. THE NUTRITION OF THE EMBRYO. 



696. In a hen's egg a very small part only of the whole 

 egg, namely, a minute collection of cells called the blastoderm, 

 is actually developed into the chick and its appendages ; by far 

 the greater part of the mass included within the egg-shell, 

 namely the ' yolk ' and the ' white,' is mere nutritive material. 

 Through the porous egg-shell the oxygen of the air has adequate 

 access to the contents within, and through the same egg-shell 

 carbonic acid can escape. The yolk and the white supply all the 

 food needed by the developing chick until it is hatched, and 

 either directly or indirectly by means of the allantoic vessels the 

 tissues of the embryo and its appendages breathe through the 

 shell. 



In the mammal the supply of yolk is insignificant; almost 

 from the first the developing ovum receives nutritive material 

 from the mother. Within the ovary the ovum is fed by the 

 cells of the Graafian follicle ; and a similar mode of feeding is 

 continued for some little time in the uterus. The repeated cell 

 division of the ovum produces a compact mass of cells, the 

 ' mulberry mass,' and this in turn is converted into the ' blasto- 

 dermic vesicle,' which consists of a cellular membrane investing 

 fluid contents ; during this conversion a considerable increase in 

 the total bulk of the ovum takes place, water and nutritive 

 material passing into the ovum from the mother, probably from 

 the cells lining the Fallopian tube. Received within the uterus 

 and covered up by the decidua, the developing embryo is sup- 

 plied with food and oxygen by the cells of the uterine mucous 

 membrane with which it lies in contact, very much in the same 

 way that the growing ovum was supplied by the cells of the 

 Graafian follicle ; and the same uterine cells carry away the 

 scanty waste matters of the embryo's nutritive activity. 



The amount of food which the embryo needs and receives is 

 at first small but continually and rapidly increases : the amount 

 of oxygen which the embryo needs is at first insignificant, but 

 the need of oxygen also increases continually and rapidly, though 

 especially during the early stages it is limited by the fact that 

 the processes going on in the embryonic tissues are largely 



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