SEC. 2. THE NUTRITION OF THE EMBRYO. 



955. 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 develop- 

 ing 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. We have seen that within the ovary the ovum 

 is fed by the cells of the Graaffian 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 

 ' blastodermic 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 supplied 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 Graaffian 

 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 synthetic, 

 directed to the building up of the tissues, and such processes con- 



