70 SPERMATOGENESIS [CH. 



of zygonema, it is generally agreed that the threads shorten 

 and thicken, and that the split dividing the two halves dis- 

 appears, in the succeeding pachytene stage. But there are 

 again two entirely different interpretations of the strepsitene 

 loops which next make their appearance. Some observers, 

 whether they believe in the parallel conjugation of threads 

 or in the longitudinal splitting, agree with the interpretation 

 adopted by AGAR that the loops in strepsinema arise by the 

 separation of the longitudinal halves of the double zygotene 

 threads; they say that the longitudinal split between the 

 two halves, which temporarily disappeared in pachynema, 

 reappears and widens, so that the two halves of the double 

 thread come apart along their whole length except at their 

 ends, thus producing elongated loops. Another school, 

 however, maintains that the strepsitene threads are still 

 double, although the split which disappeared in pachynema 

 is still invisible, and they regard the loops as consisting of 

 two threads united end to end, and each of them potentially 

 split longitudinally along its whole length. The two views 

 thus seem at first sight to differ in a point of considerable 

 importance on the question whether the reduced number 

 of double chromosomes which emerges from the synizesis 

 clumping consists of pairs united end-to-end or side-to-side. 

 Such descriptions as that of AGAR in the spermatogenesis of 

 Lepidosiren, however, provide some sort of reconciliation 

 between the two conflicting opinions, for they interpret 

 the phenomena as indicating first a side-to-side conjugation 

 (parasyndesis), followed by a separation of the united 

 threads everywhere except at their ends, with the result 

 that in strepsinema the rings each consist of two chromo- 

 somes united to each other at both ends. Although, there- 

 fore, workers of this school accept the view that there is 

 side-to-side pairing in zygonema, they do not admit, as 



