1907.] Structural Constituents of the Nucleus, etc. 453 



behaviour of those alternative characters so well termed allelomorphs by 

 Bateson. In the first place, it is difficult to escape from the inference that 

 the unit characters must be due to material primordia which are ultimately 

 responsible for their appearance, and that these several primordia have a 

 separate and persistent individuality of their own. The union of the 

 primordia at fertilisation, as judged by the results, rather resembles a 

 mechanical mixture than the formation of a chemical compound. 



Inasmuch as two sexual nuclei unite at fertilisation, each one carrying with 

 it characters of that parent from which it originated, it is clear that each 

 sexual cell or gamete must possess, in its nucleus, half the whole lot of the 

 primordia that will be contained in a fertilised egg. Furthermore, the 

 manner in which the characters are distributed amongst the members of any 

 one generation is such that no sexual cell can contain more than a single 

 individual of any given pair of allelomorphs which are present in the body 

 of the organism producing the sexual cells. This circumstance is expressed 

 by the term " Purity of gametes," and it implies that, as regards any particular 

 character, the primordium contributed by one parent only will be present in 

 a gamete, the other primordium or allelomorph having been distributed to 

 another gamete. When two gametes unite having identical primordia of any 

 character, the resulting offspring must be pure bred as regards that character, 

 and their progeny, if interbred, will keep true. But this is only so for each 

 separate character ; the different characters behave quite independently of one 

 another. Thus, if we take Aa as representing one pair of allelomorphs, and 

 B5 as representing another distinct pair, no gamete can contain A together 

 with a, or B together with b, but it can contain either member of each pair, 

 i.e., AB, Ab, aB, or ab. When two of these gametes unite, it can easily be 

 seen that the characters will be combined as I have already stated. 



The assumption as to the purity of gametes, which is in accordance with 

 the facts it attempts to explain, is in harmony with cytological observations, 

 and therefore may be accepted as representing more than a mere working 

 hypothesis. 



The clear results of statistical investigations of this kind seem to me to 

 completely dispose of a view of fertilisation, otherwise quite possible, as being 

 of the nature of a chemical union of the fusing nuclei. The facts conclusively 

 point to the inference that there is only a mechanical mixture of the 

 structural units contained in each of the sexual nuclei that take part in the 

 act of fertilisation ; the units retain their own identity, and they are again 

 sorted out, though in different combinations, in connection with the differen- 

 tiation of the sexual cells for the next generation. 



We may now turn to consider the structure of the nucleus, and enquire 



