384 THE WONDER OF LIFE 



induced by many different kinds of stimulus. To set 

 the egg dividing a mechanical stimulus such as gentle 

 brushing or a pin-prick may suffice ; or a slight disturbance 

 of the chemical composition of the sea- water may serve ; 

 or some alteration of the osmotic conditions by adding 

 something to the water ; or exposure of the eggs to certain 

 vapours or to electric discharges. The puzzling feature 

 is the diversity of effective stimulus. In many cases the 

 artificially stimulated egg divides and re- divides, but 

 eventually comes to naught. In a few cases, viable young 

 animals develop. Thus Professor Delage reared a miniature 

 sea-urchin from an unfertilized ovum, and Fritz Levy 

 reared young frogs. 



Without attempting any survey of the very striking 

 series of experiments, we may refer to two or three which 

 are particularly instructive. Winkler made an extract 

 of sea-urchin spermatozoa and put some of it in water 

 containing sea-urchin eggs. The eggs developed, and it 

 was inferred that the extract had ' fertilized ' the eggs. 

 The observation was right, the inference was wrong. For 

 Gies and Pichon showed that Winkler's results were due 

 to osmotic influence. The same results can be obtained 

 by using reagents that have nothing to do with sperm- 

 extract. Kupelwieser made the very interesting experi- 

 ment of bringing the spermatozoa of the mussel (Mytilus) 

 into contact with eggs of sea-urchins (Strongylocentrotus 

 and Echinus), with the result that the eggs developed into 

 larvae. Microscopic analysis showed that the chromo- 

 somes of the mussel spermatozoon played no part in the 

 fertilization, but that the centrosome introduced by the 

 spermatozoon took part in the cleavage process. The 

 larvae showed only maternal characters. 



