THE REALISATION OF THE NUCLEAR FACTORS 75 



are able to develop normally in spite of the fact that the nucleus 

 of each of their cells contains the lethal factor that caused the 

 death of the other tissues. The importance of work of this kind 

 lies in the fact that from the mode of operation of these lethal 

 genes, the conclusion can be drawn that the corresponding 

 processes in normal development are governed by normal genes. 

 This can be seen as follows: lethal genes have originated from 

 normal ones by mutation. Consequently, the resulting disturb- 

 ance in development must be due to the disappearance, or 

 modification, of the normal action of the gene. Indirectly, 

 therefore, these observations give us information as to the time 

 and place at which the genes exert their influence in normal 

 development. 



The genes co-operate with the cytoplasmic factors. We can- 

 not find out, simply by looking at the result, what has been 

 the part played by each in the origin of the embryo. In order to 

 assess the relative importance of cytoplasmic and nuclear 

 factors, we must combine them in such a way that the groups 

 can be told apart by their specific recognition marks. This can 

 be done by combining nuclear material of one species with egg 

 cytoplasm of another ; the properties of the embryo that results 

 from such a combination may give some information on the 

 problem under consideration. 



The crossing of two related species is the simplest way to 

 achieve this. It results in eggs, the cytoplasm of which is almost 

 entirely maternal, whereas the nucleus contains genes of two 

 species, because it arises from the fusion of egg and sperm 

 nuclei. One might expect that it would be very easy to conclude, 

 from the development of such hybrids, when and how the action 

 of the paternal genes becomes manifest in development. 



In practice, however, this type of crossing experiment is 

 fraught with certain difficulties which seriously reduce its 

 usefulness for the solution of our problem. Firstly, crosses of 

 somewhat distantly related species do not as a rule give viable 

 embryos. Broadly speaking, it can be said that the more remote 

 the two parent species are from each other, the earlier the 

 development of the hybrids will come to a stop. However, taxon- 

 omic affinity is not the only factor to be considered, as may be 



