406 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
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. 72. At A is diagrammed a “resting” nucleus at the completion of 
Fic. 72.—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. Asa 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 
