126 THE GERM-CELLS 



on the whole that of a typical cell-nucleus, but is subject to very great 

 variation, not only in different animals, but also in different stages of 

 ovarian growth. Sometimes, as in the echinoderm ovum, the chro- 

 matin forms a beautiful and regular reticulum consisting of numer- 

 ous chromatin-granules suspended in a network of linin (Fig. 58). 

 In other cases, no true reticular stage exists, the nucleus containing 

 throughout the whole period of its growth the separate daughter-chro- 

 mosomes of the preceding division (copepods, selachians, Amphibia),i 



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Fig. 58. — Ovarian egg of the sea-urchin, Toxopneustes (X 750). 

 g.v. Nucleus or germinal vesicle, containing an irregular discontinuous network of chromatin; 

 g.s. nucleolus or germinal spot, intensely stained with haematoxylin. The naked cell-body con- 

 sists of a very regular alveolar meshwork, scattered through which are numerous minute granules 

 or microsomes. {Cf. Figs. 11, 12.) Below, at s, is an entire spermatozoon shown at the same 

 enlargement (both middle-piece and flagellum are slightly exaggerated in size). 



and these chromosomes may undergo the mo.st extraordinary changes 

 of form, bulk, and staining-reaction during the growth of the o-^g? 

 It is a very interesting and important fact that during the growth 

 and maturation of the ovum a large part of the chromatin of the 

 germinal vesicle may be lost, either by passing out bodily into the 

 cytoplasm, by conversion into supernumerary or accessory nucleoli 

 which finally degenerate, or by being cast out and degenerating at 

 the time the polar bodies are formed (Figs. 97, 12^). 



The nucleolus of the egg-cell is, as elsewhere, a variable quantity 

 and is still imperfectly understood. It often attains an enormous 

 development, forming the " Keimfleck " or "germinal spot" of the 



1 p. 273. ^ p- 11^' 



