CHROMOSOMES AND MITOCHONDRIA 253 



assuming the form of paired chromosomes of various 

 shapes and sizes (Fig. 62, 10). A spindle then 

 forms; these *' heterotypic" chromosomes are 

 drawn upon it ; and each daughter cell receives one 

 chromosome of each pair. 



This mitosis is called heterotypic because it differs 

 from ordinary indirect nuclear division in two im- 

 portant respects : (1) the chromosomes are ])resent 

 in pairs, and entire chromosomes are separated, and 

 (2) the result is a reduction of chromosomes in the 

 daughter nuclei to one-half the somatic number. 

 According to certain investigators {e.g., jNIeves, 

 1907) the union of the leptotene threads in the synap- 

 tene stage (Fig. 62, ^) does not occur, but the two 

 parallel threads are simply the halves of a single 

 longitudinally split filament which fuse in the 

 pachytene stage (Fig. 62, 6-7), and separate again 

 in the diplotene stage (Fig. 62, 8-9). The large 

 majority of cytologists, however, believe that the 

 leptotene threads represent chromosomes which 

 actually fuse in pairs in the pachytene stage and sep- 

 arate from each other during the heterotypic mitosis. 

 Furthermore, the chromosomes of each pair are con- 

 sidered to be homologous, that is, the one derived 

 from the spermatozoon is morphologically similar, to 

 its mate, which is derived from the egg nucleus. 



Investigators who believe synapsis to be a fact, 

 that the conjugating elements are chromosomes, 

 and these chromosomes are identical with those of 

 the last diploid mitosis are not agreed as to the 

 method of union and subsequent separation of the 



