106 CYTOPLASMIC STRUCTURES [CH. 



there is in addition a true shell which is secreted by the 

 oviduct after the egg has left the ovary. 



The yolk consists of granules which may be of more than 

 one kind ; they may be scattered in the protoplasm with no 

 definite arrangement, or, especially when the yolk is abund- 

 ant, may be disposed in zones. The yolk-granules of some 

 species stain deeply with chromatin stains, a fact which 

 may occasionally lead to confusion in studying the nuclear 

 divisions in the egg, if care is not exercised. In a variety of 

 animals a body known as the yolk-nucleus ("Dotterkern ") 

 has been described, which may persist to a late stage of 

 segmentation as the egg develops. Its function is .uncertain 

 and there has been considerable discussion as to its origin 

 and nature. MUNSON concludes that most of the bodies 

 described as "yolk-nuclei" are in reality the persistent 

 centrospheres of the egg, though the reason for their per- 

 sistence in this way remains obscure, and further investi- 

 gation is needed. In a few cases the body is of a different 

 character, and appears to be a true nucleus which has 

 migrated into the egg from one of the surrounding follicle 

 cells. 



Some account of the so-called "organ-forming sub- 

 stances " which are present in many eggs will be given in a 

 later chapter. 



The chief points that require some consideration in the 

 development of the egg, apart from the history of the 

 nucleus which has already been described, are the deposi- 

 tion of the yolk and the formation of the egg-membranes. 

 In many species no special provision for the supply of yolk 

 to the developing egg seems to be present; the oocytes 

 gradually increase in size in the ovary and as they do so 

 yolk-granules begin to appear in their cytoplasm. In such 

 cases the substances from which the yolk is formed must 



