494 THOMAS H. BUYCM. 



the hyaloplasm flows in towards the centre. Ho points out 

 that the germ nucleus does not move till the rays of the aster 

 liave reached it, and the aster has assumed a position of 

 equilibrium towards the centre of the egg. The union is 

 thus the result of the cheraotactic forces of which the aster 

 is tlie expression. 



Wilson (1901 b) shows, liowever, that the nuclei may 

 unite in the entire absence of an astei". When eggs, im- 

 mediately after fertilisation, are placed in a weak solution of 

 cliloral (0. and R. Hertwig), or ether (Wilson), no aster is 

 developed, but when replaced in sea water the rays reappear 

 and the nuclei unite. In a certain proportion of cases, 

 which will be referred to later, the nuclei remain apai't and 

 undergo independent transformation ; but in some instances, 

 also while the eggs are still in ether, the nuclei enlarge, and 

 later conjugate in the entire absence of an aster. This 

 happens, however, only when the spermatozoon has entered 

 at a point not too far from the egg nucleus. Giardina holds 

 that this fact, and the other — that the nuclei quickly unite 

 whenever the eggs are put in pure sea water, and the aster 

 develops, — makes Wilson's observation insufficient to exclude 

 his hypothesis. Other explaiiations, such as mass attraction 

 and direct chemical attraction, both observers reject. 

 Wilson thinks the latter improbable. Again, the idea of 

 protoplasmic currents such as suggested by Butschli, 

 Erlanger, and Conklin, is not proved by actual evidence in 

 normal conditions in the sca-ui-chin egg (Wilson). The 

 changes of shape of the germ nucleus might suggest 

 amoeboid movement on its part ; but, again, this does not 

 apply to the sperm nucleus, which travels through a longer 

 path (Wilson). The changes in form might be due to the 

 exercise of chemotactic forces on the nucleus (Giardina). 



The phenomenon described by Boveri (1888) under the 

 name of ''Partial Fertilisation," has recently been worked 

 out in detail in Boveri's fixed preparations by Teichmann 

 (1902). The method by which the results were obtained 

 was that eggs which had hiin fourleen hours in unrenewed 



