THE PHYSICAL BASIS OF MENDELISM 407 



fuse along their entire length giving the figure diagrammed at C in 

 which the two loops represent two pairs of homologous chromosomes 

 in the conjugation stage, the essential step in synapsis. Following 

 this stage the two contracted loops of chromatin split lengthwise and 

 unravel in somewhat the manner shown in D. These filaments con- 

 tract 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 reduction 

 figure at F, the significant feature of which is the fact that each of the 

 daughter nuclei resulting from this division receives only two chromo- 

 somes instead of the four which the original cell at A contained. Since 

 the original cell contained one pair of larger and one pair of smaller 

 chromosomes, the daughter cells which are formed each receive one 

 larger and one smaller chromosome. 



Cytological investigation is not yet in agreement as to the inter- 

 pretation of synapsis especially as to the manner in which the phe- 

 nomena therein concerned are connected with preceding mitotic divi- 

 sions. Considering certain cytological investigations and the results 

 of research in heredity together, it appears that the threads which pair 

 in stage B represent pairs of chromosomes with homologous chromo- 

 meres occupying corresponding positions along their entire length. 

 Likewise the contraction stage at C is taken to represent a conjugation 

 of the members of pairs of chromosomes which later again separate. 

 Other cytological evidence indicates that in some forms the conjuga- 

 tion of pairs of homologous chromosomes is brought about in another 

 way. However, the essential fact is the same in either case. In the 

 reduction figure the members of each pair of chromosomes are dis- 

 tributed to the opposite poles of the spindle so that the daughter 

 nuclei received only one member of each pair. 



The significance of synapsis lies in the conjugation of homologous 

 chromosomes. In the mitoses which have preceded this particular 

 division, the chromosomes were each time conceived to be reformed 

 from the identical group of chromomeres which they contained origi- 

 nally. In synapsis, however, as shown at B there is a certain amount 

 of intertwining of the paired threads and in the unraveling of the 

 chromosomes after the contraction stage there is likewise a twisting 

 of the filaments about each other. The indications are, therefore, 

 that in synapsis there is a possibility of interchange of chromatin 

 material between the members of a pair of homologous chromosomes. 

 In all cases, however, in order to uphold our conception of the definite 



