76 MATURATION OF THE EGG [CH. 



stage have appeared identical, the bivalent chromosomes 

 contract into typical heterotype gemini and the univalent 

 members of each pair then break apart and so give rise to 

 the somatic number of single chromosomes. These latter 

 cells then develop into accessory (nutritive) cells, and never 

 form eggs. 



The true oocytes, after reaching the condition in which 

 the nucleus contains the reduced number of thick threads, 

 may undergo a very long period of growth and waiting 

 before the actual mitosis, of which the events described are 

 the preliminary stages, takes place. In an egg which when 

 mature contains much yolk the whole of it must be de- 

 posited in the interval, and while this is happening remark- 

 able changes may occur in the nucleus, intercalated, so to 

 speak, between the preliminary and final stages of the 

 division. Very commonly the chromosomes, which up to 

 this stage have been clearly defined and deeply staining, 

 disappear, and the chromatin becomes scattered through 

 the nucleus in the form of fine particles, or for a time it 

 may vanish altogether, at least in the sense that it ceases to 

 take up stain (cf. PL VIII). These changes in the nucleus 

 between the first formation of the bivalent chromosomes 

 and the division which separates them into their component 

 halves are doubtless due to the great changes that are mean- 

 while taking place in the egg. Up to the time of the emer- 

 gence of the double chromosomes from synizesis the oocyte 

 has been quite small, little larger than the corresponding 

 spermatocyte, but it now begins to increase greatly in size, 

 and, in many animals, to accumulate a large amount of 

 yolk, so that its volume may enlarge to many thousand 

 times that of the oogonium from which it has developed. 

 The nucleus, as has been seen in an earlier chapter, in some 

 way controls the metabolic activities of the cell, and its 



