46 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 
the other hand, no offspring was hatched from parthenogenetic 
eggs of either the summer or autumn brood when kept over 
winter. Von Siebold brings forward observations by himself 
and Schmid which confirm Barthélémy’s latter contention, at 
least in part. 
These investigations on silkworms are of especial interest 
to us, because we are here concerned with a transition form in 
the sense that the eggs of the silkworm show a tendency toward 
spontaneous parthenogenesis; but it depends upon certain as 
yet unknown conditions whether or not the eggs develop spon- 
taneously, i.e., without fertilization, and whether development 
stops in the initial stages, or whether the complete development 
to caterpillar or imago takes place. This is supported by the 
fact that different authors have achieved such contradictory 
results. It is possible that not only external circumstances, 
but also conditions which belong to the egg itself (e.g., the 
nature of the shell) here come into play. We shall meet later 
a similar case in the parthenogenesis of starfish eggs. 
Von Siebold, Leuckart, and other authors have extended the 
observations on natural parthenogenesis with reference to the 
importance which these observations possess for another funda- 
mental problem of biology—the determination of sex. In the 
Psychidae and in Solenobia the parthenogenetic eggs give rise 
only to females, in bees only to males. Leuckart and von 
Siebold discovered that in other hymenoptera too (Polistes, 
Vespa, and Nematus) similar things take place, and that here 
too parthenogenesis occurs, leading, however, exclusively to 
the production of males. In crustacea, such as Apus, Artemia, 
and Luimadia, von Siebold likewise found parthenogenetic 
development; but here it led to the production of females only. 
This sketch of the earlier observations upon spontaneous 
parthenogenesis will suffice for the comprehension of the ex- 
periments on artificial parthenogenesis with which they are 
historically connected. 
