62 THE PHYSICAL BASIS OF INHERITANCE 



the fact that every part of the latter may show the character- 

 istics of either or both parents " (Wilson, 1900, p. 352). 



4. Argument from Boveri's ingenious experiment. — Taking a hint 

 from the experiments of the brothers Hertwig, who showed that non- 

 nucleated fragments of unfertilised sea-urchin ova (broken by- 

 shaking) might be successfully fertilised and might segment, Boveri 

 (1889, 1895) showed that such fertilised fragments developed into 

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

 wards showed, the nuclei contain only half the normal number of 

 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 granulans) with spermatozoa of another 

 species [Echinus microluberculatus) , 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 Echino- 

 derm larvaa 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. 



In further support of the importance of the chromosomes 

 reference may be made to the fact that the number of chro- 

 mosomes in any given organism is always the same, except 

 in the reduced gametes which have half the normal number. 



Another argument may be found in the fact that in some 

 insects 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 



