76 THE REALISATION OF THE NUCLEAR FACTORS 



seen from the fact that sometimes the result of the combination 

 of eggs of species A with sperm of species B is entirely different 

 from that of the opposite combination. The arrest of devel- 

 opment may be due to different factors. Sometimes disturbances 

 occur in the nuclear divisions so that the distribution of the 

 chromosomes among the cells is abnormal. In other cases, the 

 cells show pathological phenomena from a given moment on- 

 wards. These may be restricted to certain parts or organs of 

 the germ, whereas other parts are not affected, or, on the 

 other hand, they may be present more or less equally in all 

 cells. This in itself does prove that for normal development 

 nucleus and cytoplasm must be "attuned" to each other, but 

 it makes a further analysis of the role of each group of factors 

 impossible in these cases. Hybridisation of more closely related 

 species is attended with another difficulty. The characters in 

 which these species differ do not appear until a very late stage 

 in development. During the major part of the process, the 

 hybrid is indistinguishable from either parent, and if there are 

 any influences of the paternal genes, they cannot become visible. 

 Their activity becomes manifest only at the end of development, 

 when all the more essential steps in the process have long ago 

 been completed. 



In spite of these obstacles hybridisation experiments have 

 in a few cases thrown some light on the problem under dis- 

 cussion. We shall now discuss the results of such work with 

 sea urchins, and with some species of amphibia. 



In accordance with the foregoing remarks, the result of a 

 cross between two sea urchin species depends very much on the 

 species used. In the most favourable cases, the hybrids will 

 develop into normal pluteus larvae. This will occur in particular 

 if the hybridised species are closely related, e.g. in the cross 

 between Psammechinus microtuherculatus and Paracentrotus 

 lividus. The plutei of these species are very similar, and can 

 be distinguished only by slight details in the structure of the 

 calcareous rods. In a careful investigation, Horstadius (1936) 

 obtained proof that the hybrid larvae were intermediate between 

 the parent species on this point. The same result can be seen 

 more clearly in the case of the fertilisation of eggs of Sphaer- 



