POLYEMBRYONIC DEVELOPMENT IN TATUSIA 643 



ical juggling of blastomeres to account for the various relation- 

 ships and positions assumed between the components of these 

 monsters. 



4. Polyembryony and sex 



One of the obvious biological bearings that the study of poly- 

 embryonic development has revealed concerns heredity, includ- 

 ing the heredity of sex. The illuminating studies of McClung 

 ('03), Stevens ('06), Wilson ('05-'12), Morgan ('09) and many 

 others, including the recent excellent paper on Ancyracanthus by 

 Mulsow ('12), have shown that in a large number of animal forms 

 the heredity of sex is in some way bound up with certain co-called 

 sex chomosomes, and that, as a consequence, the sex of a given 

 individual is irrevocably fixed at the time of fertilization, or in the 

 case of an. unfertilized or parthenogenetic egg, not later than the 

 time the e'gg starts to develop. 



It is in this connection that polyembryonic development fur- 

 nishes a very strong confirmation of the modern cytological view 

 of sex heredity. For in every authenic case of polyembryony 

 among dioecious species all of the individuals arising from one 

 egg are invariably of the same sex; that is they are either all males 

 or all females, never mixed. The most logical conclusion that 

 can be drawn from these facts is that the sex character is stamped 

 upon the egg prior to the origin of the several individuals to which 

 it gives rise. That the sex of the egg is determined as early as 

 the time at which development begins, seems certain in the case 

 of parasitic hymenoptera. It will be recalled that Silvestri ('06) 

 has shown in Litomastix (and he thinks that is is almost cer- 

 tainly true in Ageniaspis) that the fertilized egg produces females 

 only, while the unfertilized egg gives rise to males only, exactly 

 as in the well known case of the bee. Here fertilization or the 

 lack thereof determines the sex of the offspring, and, no matter 

 how many individuals the egg may produce by polyembryonic 

 development and in Litomastix the astonishingly large number 

 of about 3000 may develop they are all of -the same sex. 15 



15 Except, of course, the so-called asexual larvae the origin and development 

 of aaefe needf further study. 



