CORRESPONDENCE 



To THE Editor of " Science Progress " 



THE CHROMOSOME THEORY OF INHERITANCE 



From Prof. E. W. MacBride, F.R.S., D.Sc. 



Sir, — In the October number of Science Progress there appear two 

 communications from the pen of Mr. Julian Huxley. One of these is a letter 

 in reply to one of mine published in the previous number of your journal ; 

 this letter you were good enough to let me see in proof, and I have replied 

 to it in the October number. The other is an article which is described by 

 Mr. Huxley as an effort to set forth an alternative theory of inheritance to 

 that put forvvard by me in the number of the journal which appeared in 

 March 1921, 



I should like to make a few comments on the facts adduced and the 

 arguments employed in Mr. Huxley's article. 



Mr. Huxley begins by giving an able and popular account of Morgan's 

 theory of the chromosomes as the basis of inheritance, and in particular of 

 that special type of inheritance which is ordinarily described as Mendelian. 

 With much of what he says I find myself in cordial agreement. Thus, when 

 we consider that the sperm-head which carries into the egg the paternal 

 heredity is merely a condensed mass of chromosomes, it is obvious that the 

 material basis of inheritance must be in the chromosomes. Moreover, when 

 we note that when the sperm-head enters the egg it swells up and becomes 

 changed into a male pronucleus, which exhibits exactly the same number of 

 chromosomes of the same shapes as those found in the egg-nucleus, with 

 which it is about to unite, we feel impelled to believe that each chromosome 

 must have its own peculiar value, contain in fact its own special type of living 

 substance. This belief becomes strengthened when we further discover that 

 an egg can be stimulated to develop without the intervention of the sperma- 

 tozoon, and will still develop into a typical animal, since its nucleus contains 

 one sample of each kind of chromosome ; whereas if a small egg be entered 

 by two spermatozoa abnormality and death ensue, since in this case the 

 chromosomes are irregularly distributed between the four cells which result 

 from the first division, and several of these do not receive a representative of 

 each type of chromosome. 



Again, when we reflect that each cell of the body of a normal animal 

 contains two samples of each type of chromosome, one of maternal and one 

 of paternal origin, and when we consider how this number is reduced to 

 half in the ripe germ-cells, we must admit that there is great plausibility in 

 Morgan's theory that two Mendelian allelomorphs which differ from each 

 other in a single character are distinguished from one another by the fact 

 that a chromosome in one has undergone a modification which has not 

 occurred in the homologous chromosome of the other. 



But when Morgan goes further, and endeavours to prove that a particular 

 modification of character is due to a change in a special region of one chro- 

 mosome, and on the basis of linkage of characters in inheritance to construct 



4SO 



