No. 2.] ORGANIZATION OF THE EGG OF UNIO. 253 



such a way that there is no discernible relation to either 

 {Unto, Arenicola, Allolobophora, Pleurophyllidia, Planocera, My- 

 zostoma, Mus). (For the last case see Sobotta, '95.) 



It must certainly be admitted that the most recent obser- 

 vations on the fertilization and early cleavage of the ovum 

 demonstrate the inadequacy of Boveri's theory of fertilization, 

 vis. : " The ripe egg possesses all of the organs and qualities 

 necessary for division excepting the centrosome, by which 

 division is initiated. The spermatozoon, on the other hand, is 

 provided with a centrosome, but lacks the substance in which 

 this organ of division may exert its activity. Through the 

 union of the two cells in fertilization all the essential organs 

 necessary for division are brought together ; the egg now con- 

 tains a centrosome, which by its own division leads the way in 

 the embryonic development." (Quoted from Wilson, '96.) Cer- 

 tain observations, such as those of Wilson, Mead, Griffin, and 

 others, showing that the sperm-centrosomes become the cleav- 

 age centers, support it. But in other cases it is not possible 

 to trace any connection between the centers of the first 

 cleavage and the sperm-centrosomes. And it has been 

 shown by Morgan ('99) that certain chemical substances may 

 initiate cleavage of the unfertilized Qgg by stimulating the 

 egg-cytoplasm. Loeb ('99) has recently shown by an experi- 

 ment, whose conception and brilliant success must win our 

 warmest admiration, that in the egg of Arbacia not only may 

 division be initiated by adding certain chemical substances to 

 the sea water, but that the entire typical development may 

 take place under these circumstances without fertilization. 

 Neither the sperm-centrosome nor nucleus is required for the 

 typical development of the sea-urchin ^gg, which has long 

 served, on account of the clear way in which the sperm-cen- 

 ters may be shown to become the cleavage centers in normal 

 development, as the paradigm of Boveri's theory. Certainly 

 in this case the Qgg is not without the mechanism of cell divi- 

 sion, though this mechanism may usually be inhibited by the 

 more active development of the sperm-centers. If not here, 



centrosomes. These observations can thus be convincing evidence of the sper- 

 matic origin of the latter only for those whose minds are made up in advance. 



