Fasten, Parthenogenetic Data. 
63 
sexual ones. In all such cases, the progeny produced from oar- 
thenogenetic eggs may be males as well as females, whereas the 
offspring produced from fertilized eggs are all females. 
In many species, on the other hand, no alternation of gener¬ 
ation exists. Here, all the fertilized eggs produce individuals of 
one sex, the female as a rule, whereas the unfertilized eggs 
produce individuals of the opposite sex. 
Natural parthenogenesis may be said to be wholly confined to 
the invertebrate groups of animals, mainly the Rotifers, Crusta¬ 
ceans, and the Insects.—But Hertwig, Morgan and others, have 
also cited interesting instances of such partial development among 
the lower groups of backboned animals. These cases, however, 
have been seriously questioned by many workers, as will become 
evident from our later discussion of the subject. 
1. Parthenogenesis among Vertebrates. 
The eggs of many vertebrate animals show a tendency to 
divide when not fertilized. Such cases have been observed in the 
eggs of Amphioxus, the Trout, the Sturgeon, and in many of the 
Amphibia, including the Frog. In this last named case, Pfliiger 
showed that there is no actual parthenogenetic segmentation, but 
that the egg divided under the influence of spermatozoa whose 
vitality was almost entirely lost through floating about in the water. 
In the birds we get examples of parthenogenetic segmentation, 
and often a blastoderm develops from what seems to be an unfer¬ 
tilized egg. Barfurth, however, showed that in these cases also, 
segmentation is not brought about parthenogenetically, but through 
fertilization by a spermatozoon whose vitality has been almost 
wholly destroyed. 
It is thus seen, that all cases of natural parthenogenesis among 
the Vertebrates, must be looked upon with a great deal of skep¬ 
ticism, unless we are presented with more substantial evidence. 
It seems probable, however, that parthenogenesis may be induced 
through chemico-phvsical means, as has been shown by the recent 
work of Bataillon on the frog’s eggs. 
2. Parthenogenesis among the Invertebrates. 
A. Rotifers. Cohn in 1856, was the first to note cases of 
parthenogenesis amongst the Rotifers. He found that the winter 
