THE MECHANISM OF SEX-DETERMINATION 41 



to muscle, skin, gland, ganglion, or connective tissue. 

 The early germ-cells of the male, the so-called "sper- 

 matogonia," also have this same number. It is not until 

 a later stage in their development that a remarkable 

 change takes place in them. When this change occurs 

 the thread-like chromosomes unite in pairs. This is 

 the synapsis stage — the word means to fuse together. 



It is the most difficult stage to interpret in the whole 

 history of the germ-cells. In a few forms where the 

 changes that take place have been seen to best advan- 

 tage it is found that chromosomes are in the form of 

 long threads and that these threads unite in pairs to 

 make thicker threads. When the process is completed, 

 we find half as many threads as there were before. This 

 statement is not quite true. In the case of the 

 male protenor, for instance, there are twelve ordinary 

 chromosomes and one large one. The twelve unite in 

 pairs at synapsis, so that there are six double chromo- 

 somes, but the large one has no mate (Fig. 21, B). 

 When the others have united in synapsis, it has taken 

 no part in the process, hence the reduced number of 

 chromosomes in the male is seven — the seventh is 

 the sex chromosome. 



Two divisions now follow each other in rapid succes- 

 sion (Fig. 21, C, D). In the first division (C) each 

 chromosome divides - - seven go to one pole and seven 

 to the other pole. Two cells, the primary spermato- 

 cytes, are produced. Without resting, another divi- 

 sion takes place (D) in each of these two cells. It is 

 the second spermatocyte division. Each of the six 

 ordinary chromosomes divides, but the large sex chro- 

 mosome does not divide, and, lagging behind the others, 



