1 66 CAUSES OF VARIATION 



for the last two, or maturation, divisions. In this condition the 

 egg cell remains until near the time of fertilization, when the 

 process of maturation proper takes place. 



The significant details of this interesting series of changes are 

 concerned with the nucleus and are substantially as follows: 

 During the long resting stage preparatory to these final divisions 

 the nucleus increases in bulk and the chromatin matter assumes 

 the reticular form characteristic of the resting stage of dividing 

 cells in general. In this condition the nucleus is known as the 

 "germinal vesicle." Up to this point the number of chromo- 

 somes is the same as that of the body cells in general. Their 

 identity is, of course, now lost, but as the time for the first 

 maturation division arrives, instead of the spireme of ordinary 

 mitosis breaking up into the usual number of chromosomes, 

 there appear more or less spontaneously a number of "primary 

 chromatin masses " in the form of rods, rings, or V-shaped bodies, 

 each of which ultimately breaks up into four smaller bodies. 

 These groups of four are always one half the usual or 'somatic 

 number of chromosomes. 



Whether the chromatin masses appear in the form of rods, 

 rings, or otherwise, the final result seems to be always the same ; 

 namely, the breaking up of each into four smaller bodies, either 

 by two longitudinal divisions or by one (the first) longitudinal 

 and one transverse. The details differ in different species and 

 have been worked out in but few cases. It is not important 

 here to trace the bewildering differences, but rather to describe 

 typical behavior. 1 



Having assumed this condition the nucleus now migrates to 

 the margin of the cell, each of the groups of four (tetrads in rod- 

 shaped cases) splitting into two smaller groups of two each 

 (dyads). 2 The mass now divides, one pair from each group 



1 For a full discussion of the different forms of reduction, see Wilson, The 

 Cell, V, 233-287. 



2 The terms " tetrad " and " dyad " of course apply only in the case of rod- 

 shaped masses. In the case of rings (common in animals) and V-shaped masses 

 (common in plants) the parting into four takes place gradually as the work pro- 

 ceeds, while in the case of rods the division into four takes place early and the 

 parts are distinct from the first. In this formation the two divisions take place 

 much more rapidly than in the case of rings, which split and divide slowly. The 



