PRESIPENTIAL ADDRESS. 459 



specific properties might all become acciimiil;ite<I in one daughter-cell, and 

 those having opposite properties in the other, so that the two daughter-cells 

 would then differ entirely in their properties. 



I ean but refer briefly here in passing to the interesting theory put forward 

 by Butschli, to the effect that sexual phenomena owe their first origin to 

 differences between cellular organisms resulting from the imperfections of 

 the primitive methods of ceU-division. If we assume, for instance, as so many 

 Jiave done, that one of the earliest (lualitative differences between different 

 chromatinrgranules was that while some influenced more especially the trophic 

 activities of the cell, others were concerned specially with kinetic functions; 

 then it might easily happen, after nuclear division by chromidial fragmentation, 

 that all, or the majority of, the kinetic elements pass into one of the two 

 daughter-cells, while its twin-sister obtains an undue preponderance of trophic 

 chromatin. As a consequence, some cells would show strong kinetic but feeble 

 trophic energies and others the opposite condition, and in either case the 

 viability of the cells would be considerably impaired, perhaps inhibited. If 

 it be further assumed that cells of opposite tendencies, kinetic and trophic, 

 attract one another, it is easy to see that the union and fusion of two such 

 cells, the one unduly kinetic (male) in character, the other with a corresponding 

 trophic (female) bias, would restore equilibrimn and produce a normal cell with 

 kinetic and trophic functions equally balanced. On this view, sexual union, 

 at its first appearance, was a natural remedy for the disadvantages arising 

 from imperfect methods of nuclear division. 



It is not surprising, therefore, to find that the process of nuclear division 

 undergoes a progressive elaboration of mechanism which has the result of 

 ensuring that the twini sister-granules of chromatin produced by division of 

 a single granule shall be distributed between the two daughter- cells, so that 

 for every chromatin-grain obtained by one daughter-cell an exact counterpart 

 is obtained by the other ; in other words, of ensuring an exact qualitative, 

 as well as quantitative, partition of the chromatin-particles. In its perfect 

 form this type of nuclear division is known as karyokinesis or mitosis, and all 

 stages in its progressive development are to be found in the Protozoa. 



In the evolution of nuclear division by karyokinesis two distinct processes 

 are being developed and perfected in a parallel manner, but more or less 

 independently ; first, the method of the partition and distribution of the 

 chromatin-grains between the two daughter-nuclei ; secondly, the mechanism 

 whereby the actual division of the nucleus and the separation of the two 

 daughter-nuclei are effected in the cell-divisioni. I have dealt elsewhere "' 

 with the evolution of the mechanism of karyokinesis as exemplified by the 

 numerous and varied types of the process found amongst the Protozoa, and I 

 need not discuss the matter further here, but the behaviour of the chromatin- 

 grains may be dealt with briefly. The main feature in the process of the 

 exact quantitative and qualitative distribution of the daughter-chromatin 

 between the daughter-nuclei is the aggregation of the chromatin-grains or 

 chromioles into definite, highly individualised structures known as chromosomes. 

 In the most perfected forms of the process of chromosome-formation the 

 chromioles become imited into a linear series termed by Vejdovsky a chromo- 

 neme, which is supported upon a non-chromatinic basis or axis. According 

 to Vejdovsky, the supporting substance consists of linin ; R. Hertwig, however, 

 in his well-known studies on Actinosphcerium" considers that the supporting 

 and cementing substance of the chromosome is plastin derived from the 

 substance of the nucleoli. However that may be, the essential feature of the 

 chromosome is the cementing together of the chromioles to form the chromo- 

 neme, a thread of chromatin which may be disposed in various ways on the 

 supporting axis, sometimes being wound spirally round it (Vejdovsky>). 



The actual division of the chromatin takes place by the longitudinal 

 splitting of the chromoneme, in other words, by simultaneous division into two 

 of each of the chromioles of which the thread is composed. In this way every 

 chromiole which was contained in the original chromoneme is represented by 

 a daughter-chromiole in each of the two daughter-chromonemea. It follows 



" Op. rit. pp. 105-120. 



" Ahhnndl. layer. Akad. (II. CI.) xix. 1898. 



