ARTIFICIAL PAllTHENOGENESIS AND FERTILISATION. 493 



The essential difference between the pi'ocesses seen in 

 magnesium eggs and normal fertilisation is that whereas in 

 fertilisation there is only one, and that a definitely localised 

 point of astral activity, in the magnesium eggs there are a 

 number of foci, and development in large measure depends 

 on the accident of their number in the nuclear area. 



There is the same want of unity of purpose that is seen in 

 polyspermic eggs, in which the number of points of astral 

 activity depends on the number of spermatozoa which gain 

 an entrance. 



It has long been recognised that the union of the nuclei 

 and the initiation of division are co-ordinated, but in a 

 measure independent factors in fertilisation. Partheno- 

 genetic development under artificial agents is the latest 

 proof of this. The possibility of the development of 

 enucleated egg fragments when entered by a spermatozoon, 

 as described by Boveri, and afterwards named raerogoiiy by 

 Delage, is another. Either nucleus is sufiicient in itself. 



With the problems underlying the nuclear conjugation 

 this article is not concerned. It starts from the assumption 

 that the union of equivalent nuclei is the end of fertilisation, 

 but not the means (Boveri). 



The cause of the nuclear conjugation is not as yet under- 

 stood. The first possibility is tliat the aster is concerned in 

 bringing them together. Giardina (October, 1902) brings 

 the latest suggestion on this line. Starting from the basis 

 of the alveolar structure of protoplasm, he suggests that the 

 aster is the expression of both centripetal and centrifugal 

 currents. The centrosome is concerned in the diffusion of 

 chemotrophic substances into the egg, while at the same time 



structure. In auother set, in wliicli tlie centrosome had reacted differently, 

 tliere is a centriole within the centrosome, which divides before the centrosome, 

 so that it is double in the metaphase. In Wilson's earlier account there was 

 no central body, but in later descriptions there was a mass of granules in a 

 well-defined sphere, which succeeded a single granule of earlier stages. In 

 my previous paper, I regret that I misrepresented Professor Wilson's nomen- 

 clature by referring to this as his centrosome. The sphere, as a whole, is 

 named the centrosome. See note to page 314, "The cell, etc.," 1900. 



