x] CYTOLOGY 147 



an equatorial plate on the spindle, split in the typical way, 

 and the halves travel to the poles. The chromosome number 

 in the first and subsequent divisions is the reduced or 

 haploid number; although this was at one time denied, later 

 work has proved it beyond doubt, so that here, as in the 

 males of the Hymenoptera, an individual which appears to 

 be normal in every respect may exist with only half the 

 typical number of chromosomes. With regard to the origin 

 of the centrosome and aster there has been some dispute, 

 and the matter can hardly be regarded as finally settled. 

 WILSON found that small asters with a central dot could be 

 produced in non-nucleated fragments of eggs by the agents 

 which induce artificial parthenogenesis, and both he and 

 many other investigators believe that the centrosome 

 arises de novo. Others, however, maintain that a centrosome 

 persists from the polar divisions and accompanies the egg- 

 nucleus, and that it gives rise to the centrosome of the first 

 segmentation spindle in the parthenogenetic egg 1 . 



Although the greater part of the work on artificial 

 parthenogenesis has been done on Sea-urchin eggs, it has 

 been shown that the eggs of quite a large number of animals 

 are capable of being caused to develop in this way. The 

 methods used, of course, vary somewhat with different 

 species, but the essentials of the process appear to be the 

 same in all cases. The eggs of certain Starfish can be caused 

 to develop parthenogenetically even more easily than those 

 of Sea-urchins. DELAGE found that a strong solution of 

 carbon dioxide in sea-water was an efficient means, and it 

 has been shown that simple shaking is sufficient in some 

 species to induce membrane formation and development. 

 Some species may be naturally parthenogenetic, and LOEB 

 suggests that the egg-surface is in an unstable condition so 



1 E.g. PETRUNKEWITSCH (1904), BUCHNER ^1911). 



IO 2 



