220 



CYTOLOGY 



morphism, so that it was represented 

 by two parallel developmental series, 

 a male and a female. The phylogenetic 

 course along which this reduction, as 

 also the development of the dimor- 

 phism, proceeded, can be traced back- 

 wards. 



The constancy of the number of 

 the chromosomes in the nuclei of the 

 sexual cells is doubtless of great impor- 

 tance, for it ensures the equal influence 

 of the two parents in the sexual act: 

 and the act of fertilization is, in all 

 the higher organisms, the centre of 

 gravity of the maintenance and de- 

 velopment of the species. 



It is established that, in the higher 

 plants, all the nuclear divisions which 

 lead up to the formation of the sexual 

 cells are normally attended by longi- 

 tudinal splitting of the chromosomes, 

 so that the number of the chromo- 

 somes remain the same throughout. 

 There is no such thing, among plants, 

 as nuclear divisions resulting in the 

 reduction by one-half of the number 

 of the chromosomes. Such a concep- 

 tion involves the assumption that the 

 entire, not longitudinally split, chro- 

 mosomes of the mother-nucleus be- 

 come separated into two groups, each 

 of which goes to form a daughter- 

 nucleus. If this be so, then each daugh- 

 ter-nucleus must contain only half as 

 many chromosomes as the mother- 

 nucleus; and, in the next generation, 

 each nucleus must contain only half 

 as many chromosomes as a daughter- 

 nucleus: but nothing of the kind can 

 be observed among plants, a fact 

 which has to be taken into account in 

 a consideration of the phenomena of 

 heredity. 



I have rejected the view of the 

 hereditarily unequal division of nuclei 

 on the ground that it is contran- to the 

 facts ascertained bv direct observation, 

 and I am equally unable to admit that 

 theories of heredity are justified in 

 reconstructing the nucleus with the ob- 



ject of finding in it all the structures 

 which are necessary to them: the only 

 legitimate point of departure is af- 

 forded by the actually observed facts 

 of nuclear structure. I consider Weis- 

 mann's conception of the id, as an ele- 

 ment in the nucleus which is charged 

 with all the hereditary characteristics 

 of the species, to be felicitous, because 

 it appears to me that it can be sup- 

 ported by direct observation. I regard 

 as ids the discoid segments of the 

 chromosomes, which are all exactly 

 similar in form and structure, and are 

 serially arranged with such remarkable 

 regularity in the chromosomes of nu- 

 clei about to divide. 



It is by their simultaneous activity 

 that the constancy of the species is 

 proportionately maintained: for the 

 co-operation of so many ids must pro- 

 duce a resultant effect which would be 

 a mean between the individual varia- 

 tions of the successive generations. If, 

 however, in consequence of the re- 

 peated union of individuals presenting 

 a similar variation, the number of ids 

 representing this variation be increased, 

 the variation must become permanent. 



At each longitudinal splitting of the 

 chromosomes during nuclear division, 

 all the ids are halved and are equally 

 distributed to the succeeding genera- 

 tions of nuclei. The number of the ids 

 would, however, become doubled at 

 each sexual act, were it not for the re- 

 duction which takes place at the initia- 

 tion of each sexual generation. Since 

 this reduction is not due either to ex- 

 trusion or to an absorption of the 

 chromosomes, at least in plants, the 

 only remaining explanation is that it 

 is due to the fusion in pairs of the ids 

 and therefore also of the chromo- 

 somes. 



It is now known that the chromo- 

 somes of the two parents do not lose 

 their independence in connexion with 

 the sexual act. Hence in hybrids the 

 chromosomes of both father and 



