v] TERMINOLOGY 67 



the production of two daughter-nuclei (secondary sperma- 

 tocytes), each of which has half the somatic number (and 

 therefore often called the "reduced" number) of single 

 chromosomes. Almost immediately after they are formed, 

 the secondary spermatocytes begin to divide again. This 

 time the chromosomes arrange themselves in the ordinary 

 way in the equatorial plate, split longitudinally, and the 

 halves travel to the poles as in an ordinary division (PI. IV, 

 Z). This second division is therefore spoken of as a homotype 

 mitosis, and gives rise to the spermatids, which proceed to 

 develop directly into spermatozoa. 



Before proceeding to discuss how far the events here 

 described are typical of the "meiotic phase" in the matura- 

 tion of the germ-cells of animals and plants in general, it 

 will be convenient to mention certain terms which are 

 now coming into general use to describe the various stages 

 (PI. V, A-F}. The terms (mostly introduced by VON WINI- 

 WARTER of Liege, 1900) have the advantage of avoiding 

 considerable circumlocution in describing the stages of 

 maturation, but unfortunately alternative names intro- 

 duced subsequently have caused a certain amount of 

 duplication. The early stage of the primary spermatocyte, 

 when the nucleus contains a network of thin threads, is 

 called leptonema\ that when the threads unite in pairs (or 

 in any case, when they first become double) is zygonema; 

 the "bouquet stage" with thick threads is pachynema, 

 and the succeeding condition in which the thread again 

 appears double or loop-like, interpreted by AGAR as a 

 separation of the paired threads except at their ends, is 

 strep sinema or diplonema. The corresponding adjectives 

 are leptotene, zygotene, pachytene and strepsitene. Also it 



germ-cells of vertebrates and the higher plants ; it is now known in many 

 other groups. 



52 



