1907.] Structural Constituents of the Nucleus, etc. 455 



can be certainly affirmed that they are equally shared between the two 

 daughter-nuclei. Seeing, then, that the net result of the process is to 

 accurately divide the chromosomes (and consequently the chromomeres) by 

 longitudinal fission, it would seem to be immaterial whether the actual 

 numbers of these bodies remained constant or not, at any rate in the somatic 

 divisions. The uniformity so commonly observed, especially when coupled 

 with the occasional departure from it, probably indicates that the number 

 typically present is to be taken as an expression of the organising function of 

 the cell as a whole, rather than as evidence of the independent nature of the 

 chromosomes themselves.* And this point of view is intelligible when we 

 reflect that they are themselves only organised bundles of chromomeres. 



At fertilisation, two nuclei unite, and their chromosomes consequently 

 mingle. The immediate descendants of the cell thus produced are all 

 similarly provided with double the number of chromosomes contained in the 

 nuclei of the sexual cells before fusion occurred. It is, of course, clear that, 

 in any organism, a limit to such duplication must soon be reached. It is 

 found, as a matter of fact, that each act of fertilisation, with its concomitant 

 doubling of the chromosomes, is associated with a correlative process of 

 reduction to one-half. To this process the name of Meiosis} has been given. 

 The way in which meiosis is effected is surprisingly uniform in both animals 

 and plants, and its physiological importance may be gathered from the fact 

 that, whilst it occurs in every organism that reproduces itself sexually, it is 

 not bound up with morphological periods in the life history. It antedates 

 all morphological differentiation, and is only directly related to sexuality, of 

 the continuance of which it is both a consequence and a condition. 



The great theoretical interest attaching to the phenomenon of meiosis lies 

 in the convincing proof which it affords of the existence of persistent 

 structural units which are directly responsible for the appearance of the 

 various characters manifested by the developing organism. 



It was formerly held by many investigators that the significance of 

 fertilisation consisted merely in doubling the amount of chromosome- 

 substance, and that reduction merely paved the way for this addition of bulk. 

 The mass was supposed to act as a whole, rather than to be made up of 

 separate and independent units. All the results of recent investigations have 



* Much has been made of the fact that the chromosomes of successive cell generations 

 can sometimes be almost certainly identified with those that went into rest during the 

 previous telophase. But it should be remembered that such may easily happen, 

 especially when divisions succeed each other rapidly, for the chromosome substratum 

 chiefly consists of colloidal substance, which would only slowly lose its individuality under 

 quiescent conditions. 



t Farmer and Moore, * Quart. Journ. Micr. Sci., 5 vol. 48. 



