THE PHYSICAL BASIS OF MENDELISM 61 
Preceding the actual reduction division the chromatin material passes 
through a complex series of steps which may be included under the term 
synapsis. (This term is sometimes applied in a specific sense to the 
pairing of homologous chromosomes and sometimes to the contraction 
of the chromatin threads in the conjugation stage.) The essential steps 
in the prereduction process are shown in outline in Fig. 29. At A is 
diagrammed a “‘resting”’ nucleus at the completion of the multiplication 
divisions in the germ plasm. As a result of the exact type of mitosis 
which has been outlined above it contains the full number of chromosomes 
characteristic of the species. The chromatin of the nucleus next becomes 
Fic. 28.—Diagram of mitosis in a species having four chromosomes in its cells. A, The 
‘resting’ cell. B, Formation of the spireme-thread. C, Longitudinal division of the 
spireme-thread and transverse segmentation into four chromosomes. JD, Separation of the 
daughter chromosomes formed by longitudional splitting of spireme-thread. JH, Beginnings 
of nuclear reconstruction and division of the celi body. F, Cell division complete and 
daughter nuclei in the ‘‘resting”’ stage. 
‘ 
organized into threads of chromomeres which pair as shown at B. In this 
diagram the paired threads are taken to represent homologous chromo- 
somes, and the opposite chromomeres in a pair of threads are considered 
as the homologous chromomeres of the two chromosomes. The paired 
threads contract and fuse along their entire length giving the figure 
diagrammed at C in which the two loops represent two pairs of homolo- 
gous chromosomes in the conjugation stage, the essential step in synap- 
sis. Following this stage the two contracted loops of chromatin split 
lengthwise and unravel in somewhat the manner shown in D. These 
filaments contract again forming the intertwined pairs of chromosomes 
shown at E, and the nuclear membrane thereupon begins to disappear. 
Further contraction and the formation of a spindle results in the reduc- 
Digitized by Microsoft® 
