84 THE REALISATION OF THE NUCLEAR FACTORS 



determination of the shape of the calcareous rods. Lower vis- 

 cosity, which may occur as a specific character in some species, 

 and may be caused by a rise in temperature in others, leads to 

 more complicated branching of the skeleton. Therefore, the 

 primarj^ influence of the nuclear factors in the merogones 

 mentioned above might consist in a modification of the vis- 

 cosity of the protoplasm of the skeleton-building cells. The 

 activity of the nuclear factors would in that case manifest 

 itself in an influence on the physico-chemical properties of 

 certain cells of the body. 



In Amphibia, merogonic development may be brought about 

 by several means. G. and P. Hertwig killed the nucleus of the 

 egg by irradiation with mesothorium, and then fertilised with 

 normal sperms. Or, on the other hand, the nucleus may be 

 removed from the egg with a fine pipette. Finally, a method 

 often followed is to constrict the eggs with a hair soon after 

 fertilisation in such a way that one half contains the egg 

 nucleus, and the other a sperm nucleus. The latter half will 

 then develop merogonically. Homospermic merogones, i.e. mero- 

 gones in which the sperm nucleus belongs to the same species 

 as the egg cytoplasm, are often able to differentiate into a 

 harmoniously built embryo. In heterospermic merogones be- 

 tween newts of the genus Triton, development will cease at a 

 given stage, and this will be earlier, the more distant the parent 

 species. In any case, development will stop before the stage at 

 which the differential characters between the parent species 

 begin to show (Baltzer, 1920). For that reason, these mero- 

 gones themselves, again, do not help us much towards the 

 solution of our problem. Hadorn (1932) found, however, that 

 e.g. in merogones of Triton palmatus (cytoplasm) and T. cris- 

 tatus (nucleus) not all tissues are damaged to the same extent, 

 although the merogones die at an early stage. The mesenchyme 

 cells of the head die, and the muscles and nervous tissue are 

 also incapable of differentiation, but skin ectoderm, heart, noto- 

 chord, gut, and pronephros, however, are apparently unimpair- 

 ed. This shows that the antagonism between nucleus and 

 cytoplasm of different species is not manifested equally strongly 

 in all parts, but especially where the cells must achieve certain 



