460 Prof. J. B. Farmer. On the [May 13, 



elapsing between one division and another. This might well happen without 

 any change of form ; just as in some crystals, for example, certain atoms 

 or groups of atoms may be replaced without altering the crystalline form. In 

 the organic world, we know that the precise form of a particular organ may 

 be produced, although the cells of which it is made up may not be those which 

 should have entered into its composition. I need only refer to the experi- 

 ments proving that a substitution of blastomeres may take place in the early 

 stages of development of the frog's egg without affecting the subsequent 

 differentiation of the tissues. Again, I might refer to the replacement of the 

 crystalline lens, when extirpated from a salamander's eye, by a new lens 

 formed not from ectodermal, but from mesodermal, cells. And, as a matter of 

 fact, the abundant anastomoses which the linin exhibits during the earliest 

 stages of nuclear division seem to provide just that mechanical condition for 

 distribution which theory requires. This anastomosis has always been a 

 difficulty in the face of assumptions of chromosomal permanence. 



The chromosomes, then, would represent similarly organised groups of 

 chromomeres, but they would not necessarily represent permanent or per- 

 sistent structures in the sense that each one is to be looked on as being 

 invariably composed of the same chromomeres. Their constancy in form 

 and number would be the expression of organisation within the cell, and not 

 of an unchanging aggregation of the same constituents. 



The relation between the chromosomes and the origin of them from the 

 male or female parent respectively of course remains unaffected ; what 

 evidence we possess points to the conclusion that the two sets of chromosomes, 

 and consequently their chromomeres, remain distinct in all the cell-generations 

 up to meiosis.* In a number of animals and plants they reappear in two 

 distinct groups at every nuclear division during the earlier stages of ontogeny ; 

 and Blackman has shown, for the Uredinese, that the nuclear approximation 

 which corresponds to fertilisation is not really consummated until many cell- 

 generations afterwards ; not, indeed, until the meiotic phase comes on. In 

 these plants, then, the chromosomal isolation is carried to an extreme, for the 

 sexual nuclei do not unite during the somatic phase to form a single nucleus ; 

 they co-exist side by side as separate bodies, although they periodically 

 divide in a synchronous fashion. 



In suggesting the chromomeres as the agents which are responsible for the 

 production of the characters, I am aware that I may be laying myself open 

 to the charge of merely retreating from a position never very strong, and now 



* Even if it should be found that this distinctness is definitely lost in the premeiotic 

 nuclei, it would not weaken the strong evidence in favour of the shufBing of the 

 primordia, and their re-arrangement in groups of homologous pairs, at meiosis. 



