82 THE REALISATION OF THE NUCLEAR FACTORS 



cytoplasm will not be able to express itself unhampered. It is 

 always more or less masked by that of the maternal genes. 

 If a better picture of the mutual influence of nuclear and cyto- 

 plasmic factors is desired, a nucleus of species A should be 

 combined with cytoplasm of species B in the absence of nuclear 

 material from the latter. If this can be achieved, the foreign 

 nucleus will not be able to use the "mediation" of this material 

 when influencing the cytoplasm, but the two must interact dir- 

 ectly. Properties of the hybrid which are characteristic of A are 

 then undoubtedly caused by the nucleus, and B-characters equally 

 certainly by the cytoplasmic factors. Such combinations can 

 indeed be realised in a number of cases. Non-nucleated egg- 

 fragments or whole eggs, the nucleus of which has been re- 

 moved or killed, can under certain circumstances develop after 

 fertilisation with nuclear material derived only from the sperm 

 nucleus present in the cell. The general term for this pheno- 

 menon is merogony. If combined with hybrid fertilisation — 

 the sperm nucleus belonging to another species — it is called 

 heterospermic or hybrid merogony. The ideal case described 

 above is realised here. The heterospermic merogone consists of 

 cytoplasm of one species, and a nucleus of another. 



In experiments on heterospermic merogony, the same dif- 

 ficulties are generally met with as in normal hybridisation: 

 hybrid merogones in which distantly related species are com- 

 bined, are of low viability, and as a rule they soon die. It is 

 interesting to note that, just as in the case of lethal genes 

 (p. 74), this "lethality" often expresses itself in the disturbance 

 of definite developmental processes at a definite stage, whereas 

 other processes, in other organs, may take a fairly normal 

 course. The combination of more closely related species gives 

 better results, which however are not quite so interesting from 

 our point of view, because the differences between the parents 

 do not become manifest till late in development. Yet these 

 experiments furnished some results, in particular again in sea 

 urchins and amphibians. 



Horstadius (1936) used the following method for the pro- 

 duction of sea urchin merogones. With a sharp needle he re- 

 moved from the egg a small segment containing the nucleus. 



