46 PHYSICAL BASIS OF HEREDITY 



constricts between them to produce new cells (Fig. 18,/). 

 When the chromosomes have reached the pole they shorten 

 (Fig. 18, g) and appear to send out anastomosing threads. 

 Around this group of threads a new nuclear wall is formed 

 (Fig. 18, /i). All trace of the separate chromosomes is 

 now lost, but between the last stage just described and the 

 stage now to be described it is supposed that important 

 changes in the chromosomes take place. This new phase 

 is spoken of as the synapsis stage. At the beginning of 

 this stage (Fig. 18, i and j) faint indications of the chromo- 

 some appear, and soon they can be seen again (Fig. 18, A:) 

 as long thin threads whose free ends place themselves in 

 parallel pairs. The pairing of the threads continues to 

 extend inwards from the ends (Fig. 18, 1) until they have 

 united throughout the length of the loops (Fig. 19,(2-). 

 There are exactly half as many of these loops as there 

 were original chromosomes, which is expected if they have 

 united in pairs. The conjugation has been accomplished. 



During the stages that follow, the double chromosomes 

 shorten and become thicker (Fig. 19,h,c,d), and con- 

 dense into the form of tetrads (Fig. 19, e). They begin 

 to separate into halves, each half is also split lengthwise. 

 A spindle appears, and the cells divide (Fig. 19, f,g,h). 

 In each cell the chromosomes show indications of passing 

 into a resting stage, as happens after all ordinary cell 

 divisions, but before this change has gone very far a new 

 spindle appears (Fig. 19, ^), and preparations for another 

 division are rapidly made. The new division completes 

 the maturation of the sperm-cells (Fig. 19, j, k, I). Each 

 of the four cells resulting from the original sperm-mother- 

 cell differentiates into a spermatozoon. 



In one of the salamanders, Batrocoseps, the matura- 

 tion stages of the male are particularly well shown. The 

 essential stages in synapsis are shown in Fig. 20, a-d 

 as worked out by Janssens. These stages are essentially 

 the same as those of Tom,opteris. During the early multi- 

 plication stages the cells of the future testes divide by 



i 



