THE BIOLOGY OF THE CELL SURFACE 



ally never fuse, although the presence of the sperm-nucleus 

 within the egg is necessary for the egg's development. 

 Indeed, in extreme cases the cleavage-nucleus is distinctly 

 bipartite and remains so during many successive cleavage- 

 stages. In the egg of the water-flea, Cyclops, for example, 

 according to Haecker such a bipartite nucleus persists from 

 one generation to the next.^ Bipartite nuclei have been 

 described for other eggs — it shows clearly in those of the 

 large tailed amphibian, Cryptobranchus. The egg of 

 Pediculopsis presents an interesting condition: some of 

 the cells resulting from its first division after fertilization 

 contain single nuclei whilst in others each chromosome per- 

 sists as a separate entity with its own spindle. Between 

 the two extremes of complete fusion at the time when the 

 egg- and sperm-nuclei appose and the persistence of these 

 nuclei as revealed by the bipartite character of the cleavage- 

 nuclei, intermediate grades exist, even in one egg-genus.^ 

 In eggs of Rhabditis, though they must be fertilized in order 

 to develop, the sperm-nucleus remains inert and never 

 unites with that of the egg.^ Briefly, actual fusion with 

 immediate loss of identity of the egg- and sperm-nuclei is no 

 sine qua non of the fertilization-process. 



Experimentally, it can be shown that fertilization may 

 take place without the egg-nucleus. The presence of 

 either the Qgg- or sperm-nucleus alone suffices for the 

 egg's development: the former is concerned in parthe- 

 nogenesis, the latter in an Qgg whose nucleus has been 

 actually or virtually removed by experimental means. The 

 fertilization of an egg-fragment without the egg-nucleus 

 is known as merogony."* Such fertilizable fragments have 

 been obtained from eggs of sea-urchins, starfishes and 



^ Haecker, i8go. See also Heberer for literature. 



- Cf. Boveri^s observatiojis on eggs of the genus, Echinus. 



'■'' Kriiger, 191 3. 



■* Delage and others. 



180 



