62 THE PHYSICAL BASIS OF INHERITANCE 



into dwarf, but normal, larvae. In these, as T. H. Morgan (1895) 

 afterwards showed, the nuclei contain only half the normal 

 number oj chromosomes, having had only a sperm-nucleus to 

 start with. 



Interesting as this was, Boveri's further experiment was yet more 

 striking. He fertilised the enucleated egg-fragments of one species 

 of sea-urchin (Sphcerechinus granularis) with spermatozoa of another 

 species (Echinus microtuberculatus) , and obtained in a few cases dwarf 

 larvae (plutei), which showed, except as regards size, the paternal 

 characters only. Therefore he concluded that the nucleus is the 

 exclusive bearer of the hereditary qualities, for it seemed from the 

 experiment that the enucleated maternal cytoplasm had remained 

 without specific influence. 



It is admitted by Boveri himself that further experiments are 

 necessary, and it must be granted also, as has been pointed out by 

 Seeliger, Morgan, and Driesch, that in cases of hybridism, as in 

 Boveri's experiment, there may be a marked illustration of what 

 is called unilateral or preponderant inheritance. Most hybrid 

 Echinoderm larvae show maternal characters only, some show 

 paternal characters only, some show both. There is also much 

 individual variability. Thus Boveri's famous experiment affords 

 no secure basis for argument. 



5. Additional Arguments. Further evidence of the importance of 

 the chromosomes may be found in the fact that the number through- 

 out any given organism is usually the same, except in the reduced 

 gametes which have half the normal number. 



Furthermore, in the history of the gametes the chromosomes are 

 distributed in a way that corresponds to the distribution of here- 

 ditary characters in Mendelian inheritance. 



Finally, one of the steps of modern advance is the proof (De Vries, 

 Gates, T. H. Morgan, and others) that definite structural changes 

 in the body of an organism are correlated with definite changes in 

 the chromosomes of the fertilised egg-cell. 



Another argument may be found in the fact that in some cases 

 the sex of the offspring seems to depend on whether the egg is 

 fertilised by a spermatozoon with an extra " accessory chromosome " 

 or by a spermatozoon without this. 



Generally accepted Conclusion. The general conclusion from the 

 foregoing and other arguments may be illustrated by two or three 

 quotations from recognised authorities. Prof. O. Hertwig says : 



