118 NATURAL SCIENCE [August 



divisions producing the sexual-cells. In Metaphyta, there are two 

 such alternating cycles of colonial and protistoid growth, the Moss- 

 Planl <»• Fern-Scale producing the sexual cells, and the Moss-Urn or 

 Fern-Plant producing the asexual spores. In 1 8 9 1 , 1 wrote of nuclear 

 reduction : — " We may perhaps regard it as an adaptation to prevent 

 the undue multiplication of chromatomeres in the zygote, and the 

 cells produced therefrom." This view has been elaborated by Stras- 

 buro-er ; but it will be better, as we shall see, to explain it in another 

 form than his. As, normally, each nucleus exhibits on its division 

 the same number of segments that it had on its formation ; the 

 fertilised egg, oosperm, zygote, or whatever we please to call a cell 

 formed by the fusion of two, on its division will present twice the 

 number that were present in either of its two original constituents. 

 If, then, at each sexual fusion this doubling continued, the number of 

 nuclear segments in each cell would increase indefinitely in geometric 

 progression, which is, of course, out of the question: a reduction must 

 take place somewhere. This necessary reduction takes place at the 

 first resumption of protistoid multiplication. In Metaphyta, 

 where there are two such resumptions, this is obvious ; in Metozoa 

 there is only one such resumption, which coincides with the forma- 

 tion of the (protistoid) sex-cells, 1 and it is this mere coincidence 

 that gave rise to the idea that reduction was a preparation for 

 cell-fusion, instead of being the necessary consequence of 

 cell-fusion. 



A very curious case is that of Eucus, the Bladder- Wrack, which, 

 like an animal, has only one colonial form — the familiar plant, and 

 one protistoid reproduction, that producing the sexual cells ; here, 

 as we should anticipate, reduction occurs as in Metazoa at the incep- 

 tion of the latter process. Had this case been worked out before 

 that of the Vascular Cryptogams, it would have afforded great support 

 to the physiological hypothesis. 



Again, the little fresh-water Algae, the Conjugatae, have their 

 cells isolated, or at most in simple colonies of filaments, where the 

 cells, placed end to end in a single row, divide each on its own 

 account, so that they are really rather protistoid than comparable 

 with the differentiated colonial cells of higher plants. In these 

 plants nuclear reduction occurs at yet another point of the cycle, 

 namely, at the very first cell-divisions of the zygospore, which is, as 

 we see, the resumption of protistoid cell division after conjugation. 



Strasburger's statement of this explanation is somewhat dif- 

 ferent. He writes : " The morphological cause of the reduction in 

 number of the chromosomes ... is in my opinion phylogenetic. 

 I look upon these facts as indicating a return to the original 



1 I n 1,a r S x, by exce l ,tional ca ses where the reduction occurs at a very early period in 

 cells of the ovary. 



the 



