96 THE FROG. 



It consists of a thick elastic nuclear membrane, apparently per- 

 forated by fine radial pores, and inclosing a watery nuclear fluid ; 

 the latter is traversed by a finely granular protoplasmic network, 

 enlarged at the nodes to form nucleoli, or germinal spots, of 

 which one is usually larger than the others. 



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The yolk granules are small, sharply defined, spherical or 

 ovoidal, yellowish particles of food-substance, which are elabo- 

 rated by the follicle cells and passed on from them into the 

 ovum. They are confined to the protoplasm of the cell-body, 

 not penetrating into the nucleus. They increase rapidly in 

 number as the egg approaches maturity, and it is to them that 

 the size of the egg as well as its opacity are chiefly due. 



When the egg has attained a diameter of about 0'5 mm. an 

 exceedingly thin structureless investment, the vitelline mem- 

 brane, is formed immediately around it, and within the follicle. 

 The mode of origin of the vitelline membrane is not clearly 

 made out, but it seems to be formed from the egg itself rather 

 than from the follicular epithelium. 



A little later, and as the egg is approaching its full size, a 

 layer of black pigment appears on its surface ; this is at first 

 irregularly distributed over the whole surface, but, as the egg 

 ripens, the pigment becomes restricted to one half or hemisphere, 

 and the distinction between the white and black poles of the 

 egg is thus established. The pigment is contained, and appa- 

 rently formed, within the egg itself ; but it is not clear how it is 

 formed, or what purpose it fulfils. The facts, that the pigment 

 is confined to the pole of the egg which develops most rapidly, 

 and that warmth greatly increases the rate of development, 

 suggest that the pigment may facilitate development by pro- 

 moting the absorption of heat. 



2. Maturation of the Egg. 



Our knowledge of the phenomena accompanying the matura- 

 tion of the frog's egg is based almost entirelv on the researches 



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of O. Schultze, and is still in many respects imperfect. An 

 account of these changes has already been given in the intro- 

 ductory chapter, but will be repeated here in order that the 

 developmental history of the frog may be given as fully as 

 practicable. 



The process of ripening or maturation commences in an egg 



