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 present 
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., Meves, 
1907) the union of the leptotene threads in the synap- 
‘tene stage (Fig. 62, 4) 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 
