CONDITIONS OF GAMETE FORMATION 395 



eggs, as Dzierzon maintained more than sixty years ago.' The 

 queen bee is apparently capable of producing drone eggs at any 

 time, or at least repeatedly, during her life. It is conceivable that 

 all eggs produced by the queen are potentially parthenogenic and 

 so male-producing, but that when fertilized they produce females 

 (see pp. 344-45), but if the parthenogenic eggs are physiologically 

 different from the zygogenic in this case, it seems probable that the 

 former are at least slightly younger than the latter when they 

 leave the ovary. If, as suggested above, not only the physio- 

 logical age of the animal, but the conditions in the ovary determining 

 the rate of egg production — the abundance of nutrition, etc. — are 

 factors in the determination of parthenogenic and zygogenic eggs, 

 old queens may produce parthenogenic or young queens zygogenic 

 eggs under certain conditions. Only under fairly constant external 

 conditions could a definite, fixed relation between physiological 

 condition of the egg and physiological age of the parent be expected. 



In certain of the parasitic flatworms — the digenetic trematodes 

 — two or more larval generations occur between the fertilized egg 

 and the adult stage. The first of these larval generations arises 

 from the egg as a single individual which contains within its body 

 certain cells known as germ cells. Each of these germ cells develops 

 within the parent body into a larval individual of the second genera- 

 tion, and in many cases these larvae likewise contain germ cells 

 which give rise to a third larval generation : sometimes the process 

 may continue still farther, but in any case the final larval generation 

 undergoes transformation into a single adult individual and becomes 

 sexually mature. 



The germ cells in the bodies of these trematode larvae have 

 commonly been regarded as eggs, and the development of the second 

 and following larval generations as cases of parthenogenesis. The 

 observation of Gary ('09) that these germ cells resemble partheno- 

 genic eggs in giving rise to a single polar body before beginning 

 development gives further support to this view. If these cells are 

 actually parthenogenic eggs or approach such eggs in their charac- 

 teristics, their appearance during or immediately after the embryonic 



'The latest studies on the subject, Nachtsheim, '13, Armbruster, '13, give an 

 extensive bibliography. 



