74 THE REALISATION OF THE NUCLEAR FACTORS 



Later investigations have shown, indeed, that absence of one 

 chromosome, or even of a small part of a chromosome, is 

 sufficient to bring development to a stop at an early stage. 

 Poulson (1940-45), for instance, studied the consequences of 

 "deficiencies" of varying extent in the X-chromosome of the 

 fruit fly, Drosophila. Complete absence of this chromosome 

 resulted in disturbances as early as the end of cleavage, at the 

 moment when the cleavage nuclei migrate to the surface of the 

 egg (cf- p. 70). In the case of absence of one particular half 

 of the X-chromosome the disturbances also became manifest 

 at the same stage, but the distribution of the cleavage nuclei 

 over the surface layer was slightly less abnormal. If the other 

 half of the chromosome was absent instead, the superficial 

 cell layer developed normally, but no embryo was formed. 

 Finally, absence of small parts of the chromosome led to certain 

 characteristic aberrations at later stages, e.g. in the develop- 

 ment of the nervous system or of the gut. 



These investigations prove that the full complement of 

 chromosomes is necessary for normal development. Since the 

 absence of very small parts of a chromosome may result in the 

 disturbance of very definite developmental processes, it seems 

 probable that the normal course of these processes depends 

 on the presence of special genes, located in these parts. 



We owe our insight into the way in which the genes work 

 in the first place to a number of investigations on so-called 

 lethal genes, i.e. genes which lead to the death of the embryo 

 at some given moment during its development. Sometimes these 

 genes take effect even when heterozygous, in other cases only 

 if homozygous. Each of them was found to exert its influence 

 at one definite stage of development, and often also at one defi- 

 nite place, by directing the course of development of some organs 

 or parts of the embryo into abnormal channels, leading to the 

 death of the embryo. In many cases, however, one gene proved 

 to influence several developmental processes, localised in differ- 

 ent parts of the germ, at the same time. Side by side with the 

 dying tissues, other organs on which the lethal factor has no 

 influence often occur in the same embryo. If such tissues are 

 removed in good time, and grafted into a healthy embryo, they 



