XIIl] 



ABNORMAL NUMBERS 



191 



division. And the various kinds of exceptions to the rule 

 provide strong arguments in favour of the individuality 

 hypothesis, for all, or nearly all, are easily explained by its 

 aid, but are quite inexplicable otherwise. For example, 

 BOVERI points out that occasionally in Ascaris megalo- 

 cephala univalens an abnormal polar division may leave two 

 chromosomes in the mature egg instead of one. Such an egg, 

 fertilised by a normal spermatozoon with one chromosome, 

 contains three chromosomes in its zygote-nucleus. If the 

 chromosome number were determined by some inherent 



Po 



FIG. 23. Segmentation of a three-chromosome egg of Ascaris, showing that the 

 abnormal chromosome number persists to the gastrula stage. The large 

 cell in the later embryo is the primitive germ-cell ; Po, the polar body. 

 From BOVERI (1903 a). 



character of the species, it would be expected that in the 

 later segmentation divisions the chromatin would be aggre- 

 gated into the two chromosomes which are characteristic 

 of normal embryos of this species, though each might per- 

 haps be rather larger than usual. But in fact it is found that 

 such eggs continue to show three chromosomes in the cells 

 of the germ-track (which do not undergo diminution) up to 

 the end of the segmentation stages ; when once the number 

 has become abnormal through any cause, it remains so al- 

 though the development is otherwise typical (Text-fig. 23). 



