239 



will subsequently appear, the known facts readily allow of such an 

 interpretation. 



The Reducing Division in Metazoan Reproduction. 



By J. Beabd and J. A. Mueeat, B. Sc. 



A reducing division in itself, apart from the previous history of 

 the cell in which it occurs, or of the ancestors of that cell, is of 

 course unintelligible. It is needful to enquire in both animal and 

 plant how from this past history the reduction was rendered impera- 

 tive. It is, as Strasburger has insisted, 'a return to the original 

 generation from which, after it had attained sexual differentiation, 

 offspring was developed having a double number of chromosomes" ^). 

 Theoretically it is the undoing of the displacement of balance among 

 the ''organs" of a cell due to duplication at a previous conjugation^). 



In the researches of recent years on the mode in which the re- 

 duction takes place in oogenesis and spermatogenesis the burning 

 question has been whether it was by a longitudinal, or by a trans- 

 verse, fission of chromosomes. A longitudinal division is proved to 

 be incapable of effecting this, because it is the mode in which any 

 ordinary cell-division is brought to pass. And the failure of the 

 chromosomes to unite after the conjugation, until the first division 

 of the zygote shall have happened, is again an indication that a lon- 

 gitudinal splitting does not bring about a reduction. When actual 

 union of chromosomes after conjugation is effected, this is obtained 

 by the union of the chromosomes from an individual A. with a cor- 

 responding number of chromosomes of an " individual B. ; the chromo- 

 somes must, as others have often enough insisted, retain their identity 

 and they only become disunited for the purposes of a cell-division. 



Ultimately it becomes necessary to finally undo the linking, in 

 order to prevent a duplication of the number which would increase it 

 to fourfold what it originally was. This can only be effected by a 

 transverse splitting. In other words chromosomes may be considered 

 as possessing two axes, along one of which (the longitudinal) they 

 may divide, along the other (the transverse) they may unite with 

 other chromosomes. It is along the latter — that along which union 



1) loc, cit., p. 289. 



2) The term "conjugation" is used to represent generally the union 

 of two nuclei whether in Protozoan or Metazoan. The final act of union 

 is fundamentally the same in both cases, as will appear subsequently. 



