92 



I. TJie Nucleus 



THE GERM-CELLS 



The nucleus or germinal vesicle occupies at first a central or nearly 

 central position, though it shows in some cases a distinct eccentricity 

 even in its earliest stages. As the growth of the &gg proceeds, the 

 eccentricity often becomes more marked, and the nucleus may thus 

 come to lie very near the periphery. In some cases, however, the 

 peripheral movement of the germinal vesicle occurs only a very short 

 time before the final stages of maturation, which may coincide with 

 the time of fertilization. Its form is typically that of a spherical sac, 

 surrounded by a very distinct membrane (Fig. 42); but during the 

 growth of the egg it may become irregular or even amoeboid (Fig. 58), 

 and, as Korschelt has shown in the case of insect-eggs, may move 

 through the cytoplasm towards the source of food. Its structure is 

 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. 42). 

 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),^ 

 and these chromosomes may undergo the most extraordinary changes 

 of form, bulk, and staining-reaction during the growth of the egg.^ 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 (p. 177). 



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

 quantity and is still imperfectly understood. The nucleoli are of 

 two different kinds, either or both of which may be present. One 

 of these, the so-called principal nucleolus, is a rounded, usually single 

 body, staining intensely with the same dyes that colour the chromatin, 

 and often containing one or more vacuoles. This is typically shown 

 in the echinoderm egg, in the eggs of many annelids, mollusks, and 

 coelenterates, in some Crustacea, in mammals, and in some other 

 cases. From its staining-reaction this type of nucleolus appears 

 to correspond in a chemical sense not with the "true nucleoli" of 

 tissue-cells, but with the net-knots or karyosomes, such as the nucle- 

 oli of nerve-cells and of many gland-cells and epithelial cells. The 



1 p. 193. ^ P- 245 



