v] MEIOTIC PHASE 63 



four spermatids, and each spermatid develops into a sper- 

 matozoon. Every spermatozoon, therefore, is formed from 

 a cell the nucleus of which contained half the number of 

 chromosomes characteristic of the species. 



The whole series of events from the beginning of the 

 growth-phase of the primary spermatocyte to the produc- 

 tion of the spermatids is now generally known by the term 

 meiotic phase, introduced by FARMER and MOORE (1905), and 

 it will now be necessary to consider the processes involved in 

 rather greater detail. As there is still considerable diverg- 

 ence of opinion, not only in the interpretation of individual 

 cases, but also on the question whether the phenomena are 

 really comparable in different animals, it will be advisable 

 first to describe one specific case, and a convenient example 

 to choose, both from the excellence of the work and the 

 clearness with which the various stages are seen, is the 

 spermatogenesis of the American Lung-fish, Lepidosiren 

 paradoxa, described by AGAR (1911) (PI. IV). 



In the spermatogonial nucleus the chromatin is distri- 

 buted in coarse masses over a dense network of threads 

 which fills the nucleus, and the nucleus of the primary 

 spermatocyte at the end of the growth-phase is somewhat 

 similar, differing chiefly in the chromatin being spread out 

 in threads (PL IV, A}. As the division approaches, these 

 threads become finer and more regular and distinct, but 

 have no definite arrangement (B). Then at one pole of the 

 nucleus (in many animals, and probably in Lepidosiren, at 

 the pole nearest to the centrosome) the threads take up a 

 parallel arrangement for part of their course, while remain- 

 ing as a tangled network at the opposite pole. The parallel 

 parts of the threads now begin to fuse in pairs, often be- 

 coming spirally twisted round each other in the process 

 (C, Z)). In this way thick double threads are produced 



