72 SPERMATOGENESIS [CH. 



composed of two members united end-to-end, and that the 

 separation of the members of the pair does not take place 

 until the second division. In rare cases this may possibly 

 be true, but it is now generally agreed that the first sper- 

 matocyte mitosis is normally the "reducing" division, that 

 is, the division at which complete chromosomes segregate 

 from each other, and that in the second division the chromo- 

 somes split longitudinally as in an ordinary somatic mitosis. 

 The split which takes effect in the second division is often 

 visible in the anaphase of the first or, according to some 

 observers, even considerably earlier. 



One further point about the origin of the gemini requires 

 notice. In a number of Arthropods and Worms the emer- 

 gence of the somatic number of chromosomes, each longi- 

 tudinally split, has been described as following synizesis, 

 and it is said that these then pair end-to-end, contract, and 

 give rise to gemini which, in consequence of the split in each 

 member, appear four-fold, and have hence been called tetrads. 

 In other cases, especially in Copepods, the reduced number 

 of longitudinally split chromosomes appears after synizesis ; 

 each has a transverse break in the middle, and they have 

 therefore been regarded as being pairs of longitudinally split 

 single chromosomes united end-to-end, the break correspond- 

 ing with the point of union (PI. VII). As in the preceding 

 case, they contract and form tetrads. In the first spermato- 

 cyte division the tetrads each divide into two "dyads" 

 (interpreted as single split chromosomes) and in the second 

 division this split takes effect and divides the chromosomes 

 into their component halves. Until some fifteen years ago 

 this method of reduction by means of tetrads was regarded 

 as almost universal, but more recent work has thrown grave 

 doubt on the correctness of the interpretation. As has been 

 seen above, the gemini formed by the second pairing in 



