ARTICLES 437 



This aspect of the problem is of importance because of its 

 bearing upon two fundamental cytological theories — the in- 

 dividuality of the chromosomes, and the sex chromosomes — 

 those peculiar bodies which appear to be closely connected with 

 sex-determination. It will be convenient, therefore, at this 

 stage briefly to summarise the salient points upon which these 

 two hypotheses are built, leaving to a future article the 

 discussion as to how the cytological investigations on artificially 

 parthenogenetic eggs affect the two theories. For a full 

 discussion of the theory of chromosome individuality and sex- 

 determination the reader is referred to the two excellent books 

 by the late Prof. Doncaster, An Introduction to Cytology and 

 The Determination of Sex, both published by the Cambridge 

 University Press. 



Individuality of the Chromosomes. — This theory, largely due 

 to the classical work of Boveri, is accepted by most cytologists, 

 though dissentient views are not rare. The early upholders 

 of the theory considered the chromosomes as entities, which 

 persisted without suffering any change from one cell division 

 to another. This extreme view, as it may be termed, has had 

 to be abandoned, research having demonstrated that in many 

 cases the chromosomes do not persist in this way, but may 

 become broken up into smaller units. The eggs of Ascaris 

 afford a good example of this. In the fertilised egg there are 

 two long chromosomes, which, during the subsequent cell 

 cleavage, lose their thickened ends, and in those cells, not 

 destined to give rise to the germ tract, the rest of the body 

 of the chromosomes becomes resolved into small granules. 

 There are other reasons also for believing that chromosomes 

 cannot be regarded as units. 



To meet these facts, the original hypothesis has been replaced 

 by one regarding the chromosomes as composed of small 

 portions, to each of which individuality is assigned. 



In this modified form the theory has many experimental 

 facts to support it, and the chief ones may be mentioned here. 



It has long been known to biologists that, for a given species 

 of animal and plant, the number of chromosomes is a constant. 

 To this rule there are exceptions tending, however, to support 

 the hypothesis of individuality rather than otherwise. Thus 

 the study of abnormal variations in the chromosome number in 

 the egg of Ascaris gives, to my mind, an extremely strong 

 proof of the hypothesis. Sometimes during the second 

 maturation division one or more of the chromosomes, destined 

 to be extruded with the polar body, is accidentally left in the 

 egg. Such chromosomes then form a normal reticulate nucleus, 

 and, at a later period, in this nucleus there appear as many 

 chromosomes as went to its original formation. If there was 



