408 D. J. SCOURFIELD OX MENDELISM AND MICROSCOPY. 



It would be out of place here to enter upon even a brief account 

 of the long series of researches by which it was demonstrated 

 that the actual material bearers of the hereditary tendencies 

 were the minute, highly stainable bodies existing within each 

 cell-nucleus, and known as the chromosomes. The elaborate 

 mechanism which comes into play in the process of cell-division 

 (mitosis), by means of which the two daughter-nuclei are furnished 

 with exactly half of each chromosome of the mother-nucleus, points 

 at once to the extreme importance of these bodies ; and the fact 

 that, however diverse the male and female germ-cells of any 

 species may be in other respects, they are, with very few exceptions, 

 exactly similar in respect to their chromosomes, seems to make it 

 sufficiently clear that the latter are the all-essential constituents 

 of the cells, and therefore the bearers of the hereditary factors.* 

 The answer to the above question consequently turns upon the 

 correspondence or otherwise of the behaviour of the chromosomes 

 with the Mendelian conception of segregation. 



Owing largely, I think, to the Weismannian view, which 

 prevailed in 1900, that the chromosomes were practically of equal 

 value — i.e. that each one in a particular germ-cell possessed the 

 potentiality of building up a complete organism agreeing in all 

 essential particulars with the type of the species to which it 

 belonged — there seemed to be some difficulty at first in making 

 such a correlation. But in the last few years a number of facts 

 have come to light which seem clearly to show that the different 

 chromosomes in a cell are not all essentially alike — only two, in 

 fact, being equivalent to one another in each body-cell. Coupled 

 with other recent observations on the continuity of the individual 

 chromosomes, etc., this view has rendered it possible to fit in the 

 idea of gametic purity with cytological processes in a very feasible, 

 if not absolutely convincing, way. The principal conceptions 

 about the chromosomes which are necessary to connect them with 

 Mendel's law are as follows : — 



1. The continuity of the individual chromosomes. — The proof of 

 this is not absolute, and perhaps never will be, owing to the fact 

 that except for a short time before and after cell- division the 

 chromosomes are unrecognisable as definite structures, being, as 



There are still some investigators who do not consider this question 

 closed, but most authorities regard the arguments in favour of the 

 chromosomes, and these alone, as overwhelming. 



