1918] Alternation axd Parthenogenesis in Padina 01 



The term parthenogenesis as ordinarlv understood implies the pro- 

 duction of mature individuals from unfertilized eggs. For the pur- 

 pose of clear thinking it would, therefore, seem to be necessary to 

 separate the structures usually called eggs into two groups: (1) those 

 in which there is the haploid number of chromosomes, (2) those in 

 which the diploid number ol)tains. There is a considerable number 

 of forms both in the' animal and the plant kingdom in which a so- 

 called parthenogenesis seems to be a more or less normal or natural 

 process. But so far as the writer has been able to ascertain, even 

 after diligent search, there is not a single clearly demonstrated case 

 of natural parthenogenesis among these in which the parthenogenetic 

 egg unmistakably presents the haploid or reduced number of chromo- 

 somes. In the case of the honey-bee the male is produced partheno- 

 genetically from an egg apparently having the haploid number of 

 chromosomes. However, maturation is here very abnormal. Fur 

 example, no chromatin is thrown out of the egg in polar l)(>dy forma- 

 tion, although it is separated from the female pronucleus. The fate 

 of this chromatin is, furthermore, variously interpreted. "With such 

 evidence before us, it would seem that for the present at least we are 

 warranted in refusing to consider this case as sufficiently well demon- 

 strated to be counted an exception to the general statement made 

 above. (See Hegner: "Germ Cell Cycle in Animals," p. 205.) In 

 the cases, then, of so-called parthenogenesis the structures involved 

 are not true eggs and therefore should perhaps receive a iKinic wliich 

 more accurately deiines them. However, it is with eggs having the 

 reduced number of chromosomes that artificial parthenogenesis con- 

 cerns itself. Loeb and a numl)er of other experimentalists have suc- 

 ceeded in initiating cell division in several different animal and plant 

 (Overton (0)) eggs which })resumal)]y had the rcdnced nnmber and 

 were apparently incaj^able of dividing normally. Ihe methods em- 

 ployed consist in sub]'(>('ting tliese eggs to various mechanical and 

 chemical ti-catnicnts. Tlicy iuivc tlins been able !<• caiTv tlicni t'roni 

 a few divisions to highly developed ami aii|)arently normal larva-—- 

 in the case of tln^ frog to n stage in which the tail had practically 

 been absorbed and 2 to 1 legs develojicd. I.oeb and IJancndt 



