54 THE PHYSICAL BASIS OF INHERITANCE 



nucleus and its companion- body the. centrosome, the other 

 being necessarily simply half of the living matter of the egg 

 without any nucleus. Beside them he placed an intact ovum, and 

 then let the spermatozoa in. All the three objects showed equal 

 " sexual attraction " in respect to the spermatozoa ; all three 

 were fertilised ; all three segmented, the intact ovum most 

 rapidly, the nucleated fragment more slowly, the non-nucleated 

 fragment more slowly still. In one case the development 

 proceeded for three days ; the intact ovum had become a typical 

 gastrula (two-layered embryo), the nucleated fragment a smaller 

 gastrula, and the non-nucleated fragment also a gastrula but 

 with a very much reduced cavity. All the cells of these embryos 

 showed nuclei. Thus the experimenter was led to the conclusion 

 that fertilisation and some measure of development may occur in 

 a fragment of ovum without nucleus or centrosome. The nucleus 

 of the spermatozoon must have been in this case sufficient in 

 itself, though it will be noticed that in the experiment cited the 

 fragment did not develop far. Delage makes the important 

 suggestion that in fertilisation two things must be distinguished : 

 (a) the stimulus given to the ovum by some specially energetic 

 substance brought in by the spermatozoon, perhaps in its centro- 

 some ; and (b) the mingling of heritable characteristics, Weis- 

 mann's " amphimixis." 



In subsequent experiments Prof. Delage (1899) reached even 

 more extraordinary results. Non-nucleated fragments of the 

 ovum of Echinus (sea-urchin), Dentalium (elephant's-tooth sheii), 

 and Lanice conchilega (a seashore worm), were effectively fer- 

 tilised and gave rise to the characteristic larval forms — pluteus, 

 veliger, and trochophore respectively. Three larvae were 

 reared from one ovum of a sea-urchin ; a normal blastula 

 embryo (a hollow ball of cells) was reared from ^-th of a 

 sea-urchin ovum ; a non-nucleated fragment of a sea-urchin 

 ovum, after fertilisation by a spermatozoon with nine chromo- 

 somes (nuclear rods), gave rise to a larva whose cells had the 



