THE PHYSICAL BASIS OF MENDELISM 



365 



number of chromosomes to one-half that characteristic of the somatic 

 cells. Preceding the actual reduction division the chromatin 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 con- 

 traction of the chromatin threads in the conjugation stage.) The 

 essential steps in the prereduction process are shown in outline in 

 Fig. 62. At A is diagrammed a " resting " nucleus at the completion of 



Fig. 62. — The reduction division as represented for a species whose diploid 

 number is four. A, "resting" nucleus of a primary germ cell; B, formation of 

 paired threads of chromomeres; C, conjugation of homologous chromosomes 

 (synapsis) ; D, loosening of the synaptic knots; E, condensation of the chromosomes 

 and disappearance of the nuclear membrane; F, homologous chromosomes about 

 to pass to opposite poles, thus giving each secondary germ cell a member of each 

 pair and one-half the somatic number. {From Babcock and Clausen.) 



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 num- 

 ber of chromosomes characteristic of the species. The chromatin of 

 the nucleus next becomes organized into threads of chromomeres 

 which pair as shown at B. In this diagram the paired threads are 

 taken to represent homologous chromosomes, and the opposite chro- 

 momeres of the two chromosomes. The paired threads contract and 



