140 George Lefevre 



2 Origin of the Cleavage Centrosomes 



The formation de novo of the centrosome, at first rejected by 

 Boveri ('01) but later accepted by him ('02, p. 40) on the evidence 

 furnished by Wilson's experiments ('01), has been recently at- 

 tacked by Petrunkewitsch ('04), who has attempted to defend the 

 continuity of the centrosome while gratuitously assuming that the 

 ovocenter, although invisible and undiscoverable after maturation, 

 persists in the parthenogenetic egg and later gives rise to the cleav- 

 age centrosomes. His contention that the centrosomes of the 

 multiple-asters are not new formations but arise by division of the 

 primary egg center, as do all asters containing central bodies, and, 

 furthermore, that the cytasters of egg fragments do not possess 

 central bodies, has been adequately criticised by Wilson ('04) 

 and shown to be utterly unsupported by evidence. The obser- 

 vations of both Wilson ('01) and Yatsu ('04, '05), that asters 

 containing centrosomes can be artificially induced in egg fragments, 

 in which there is no possibility of the presence of an egg center, 

 completely sets at rest the question of their formation de 

 novo, and in the light of these facts the probability of a similar 

 origin of the cleavage centrosomes in parthenogenetic eggs seems 

 to me to be very great. Since it has been experimentally proven 

 that centrosomes may be induced as new formations, it is very 

 difficult for me to conceive of the centrosome and its associated 

 radiations as anything more than an expression in cell substances 

 of forces or activities tending to produce cell division; that is, as an 

 effect rather than a cause — an opinion, I believe, which is becom- 

 ing more generally prevalent. With the destruction of the older 

 conception of the centrosome as a persistent cell organ, any attempt 

 to rescue even a shred of the former theory, in maintaining a 

 physiological unity for the centrosome as an active stimulating 

 agent in cell division, must, it seems to me, be futile. 



The independent origin of the cleavage centrosomes in the 

 parthenogenetic egg of Thalassema suggests the possibility that 

 in the normally fertilized egg they may not be derived from the 

 sperm center, but that they, too, arise as new formations in the 

 cytoplasm. Since Griffin's work on this egg, Thalassema has 



