THE PHYSICAL BASIS OF MEN DELI SM 



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 



FIG. 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. D, Separation of the 

 daughter chromosomes formed by longitudional splitting of spireme-thread. E, Beginnings 

 of nuclear reconstruction and division of the cell 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 chromomeresin 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- 



