250 GERM-CELL CYCLE IN ANIMALS 



Certain cellular phenomena which concern the 

 chromosome cycle have been described in preceding 

 chapters and so need only be mentioned here. First, 

 the occurrence of amitosis in the multiplication of 

 the germ cells has an intimate relation to the speci- 

 ficity of the chromosomes, since if nuclei divide en 

 masse it seems improbable that the chromosomes be- 

 come equally divided between the daughter nuclei 

 (see Chapter V, p. 133) ; and second, the formation of 

 nurse cells from oogonia may be accompanied, as 

 in Dytiscus (Chapter V, p. 120), by a chromatin- 

 diminution process which may be regarded as a 

 differentiation of mother germ cells into somatic 

 cells (nurse cells) and oogonia, a differentiation re- 

 sembling the segregation of the primordial germ 

 cells in the cleavage stages of the egg. 



The most striking and perhaps the most important 

 stages in the chromosome cycle occur during the 

 growth and maturation periods of the germ cells. 

 As briefly described and figured in Chapter II, 

 the mitoses which occur during maturation are 

 meiotic, since the mature germ cells have their chro- 

 mosome number reduced one-half. The events in this 

 process most worthy of our attention are those which 

 take place during the stages known as synapsis 

 and reduction. Wilson (1912) has summed up the 

 questions that remain to be solved in the following 

 words: "The cytological problem of synapsis and 

 reduction involves four principal questions, as 

 follows : (1) Is synapsis a fact ? Do the chromatin- 

 elements actually conjugate or otherwise become 



