CHAPTER XIII 



DEVELOPMENT OF GALLUS (THE CHICK) 



Fertilisation. — The true egg of the hen is all that is contained 

 within the membrane that just surrounds the yolk. It is 

 therefore of relatively enormous size for a single cell, and this 

 is due to the very large quantity of yolk which it contains. 

 The pure protoplasm, of which there is comparatively little, 

 is situated at the animal pole, which is the point at which the 

 follicle-stalk is attached to the ovary. The egg is surrounded 

 by the vitelline membrane which it has secreted and which 

 thickens to form the zona radiata, perforated by numerous 

 holes through which nutriment is passed to the egg from the 

 surrounding follicle-cells. This is a primary membrane. The 

 egg bursts out of its follicle into the ccelom, and the follicle 

 is left behind. There is therefore no secondary membrane. 

 The nucleus has grown to a very large size, and the first polar 

 body is formed inside the mouth of the oviduct, which as it 

 were grasps the follicle containing the egg before the latter has 

 left the ovary (or been " ovulated "). 



Sperms are introduced into the cloaca of the female during 

 copulation, and they make their way up to the top of the oviduct 

 where several of them penetrate an egg. After the second 

 polar body has been formed, one of these sperm-nuclei fuses 

 with that of the egg, and the other sperms degenerate. 



The fertilised egg then begins to develop, and passes 

 down the oviduct. The walls of the latter secrete the tertiary 

 membranes round it in the form of a layer of albumen, an inner 

 and an outer shell-membrane, a hard shell formed by depositing 

 lime salts, and this in many birds is coated with a layer of 



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