24& KEDL'CTIOX OF THE CHROMOSOMES 



he called ^<///<7/ ^//rvV/^v/ ( Aequationsthcilung), was then a known fact. 

 The second form, at that time a |)urely theoretical postulate, he as- 

 sumed to be of such a character that each daughter-nucleus should 

 receive only half the number of ancestral germ-plasms possessed 

 bv the mother-nucleus. This he termed a nduciiii^ division (Re- 

 duktionstheilung), and suggested that this might be effected either 

 by a transverse division of the chromosomes, or by the elimination of 

 entire chromosomes without division.^ By either method the number 

 of " ids " would be reduced ; and Weismann argued that such reduc- 

 ing divisions must be involved in the formation of the polar bodies, 

 and in the parallel phenomena of spermatogenesis. 



The fulfilment of Weismann's prediction is one of the most inter- 

 esting results of recent cytological research. It has been demon- 

 strated, in a manner which seems to be incontrovertible, that the 

 reducing divisions postulated by Weismann actually occur, though 

 not precisely in the manner conceived by him. Unfortunately for 

 the general theory, however, transverse divisions have been cer- 

 tainly determined in only a few types, while in others, of which 

 Ascaris is the best-known example, the facts thus far known seem 

 clearly opposed to the assumption. On the whole, the evidence of 

 reducing divisions, i.e. such as involve a transverse and not a longi- 

 tudinal division of the chromatin-thread, has steadily increased ; but 

 it remains quite an open question whether they have the significance 

 attributed to them by Weismann. 



B. Origin of the Tetrads 



I. Goicral Sketch 



In considering the origin of the tetrads or their equivalents, it 

 should be borne in mind that true tetrad-formation, as described 

 above, has only been certainly observed in a few groups (most 

 clearly in the nematodes and arthropods). But even in cases where 

 the chromatin does not condense into actual tetrads these bodies 

 are represented by chromosomes in the form of rings, crosses, and 

 the like, which are closely similar, and doubtless equivalent, to those 

 from which actual tetrads arise, and present us with the same prob- 

 lems. With a few apparent exceptions, described hereafter, the 

 tetrads of their equivalents always arise by a double division of a 

 single primary chromatin-rod or mass. Nearly all observers agree 

 further that the number of primary rods at their first appearance in 

 the germinal vesicle or in the spermatocyte-nucleus is one-Jialf the 

 tisiial ninnber of chromosomes, and that this numerical reduction is 

 due to the fact that the spireme-thread segments into one-half the 



^ Essay VI., ]i. 575. 



