v] DOUBLING OF THREADS 69 



however, the disagreement arises not from difference of 

 material, but from divergent interpretations of the same 

 phenomenon, and it will be necessary to devote a short 

 space to the consideration of some of those points which 

 have been subjects of controversy, in consequence of their 

 theoretical importance. 



One of the phenomena which has led to the greatest 

 amount of discussion is that of the production of the double 

 threads of the "bouquet" stage from the thin single threads 

 of the early first spermatocyte nucleus. In Lepidosiren, 

 according to AGAR, the double threads are produced quite 

 clearly by side-to-side union of the single threads seen at 

 the rather earlier leptotene stage (PI. VI, h, i) 9 and at the 

 present time the opinion that this is the true account is 

 held by an increasing number of cytologists (for example, 

 A. and K. E. SCHREINER, T. H. MONTGOMERY, E. B. 

 WILSON, A. BOLLES LEE, etc.). Until recently, however, 

 the generally accepted view was that the doubling of the 

 threads in the zygotene stage was due, not to the parallel 

 conjugation of the single leptotene threads, but to their 

 longitudinal division, and this opinion is still held by a 

 number of leading cytologists (PI. VI, a-e). They main- 

 tain that the leptotene threads thicken and then divide; 

 the threads can often be seen to consist of a row of granules 

 (microsomes) like a string of beads, and it is said that the 

 granules increase in size in a direction transverse to the 

 thread, and then each divides into two, thus giving rise to 

 a longitudinal split along the whole thread. Some of those, 

 on the other hand, who believe in the parallel conjugation 

 of the threads, maintain that this is brought about by con- 

 jugation of the granules, each granule of one thread uniting 

 with the corresponding granule of its neighbour. Whichever 

 view is adopted in regard to the origin of the double threads 



