1903.] PHILLIPS—A REVIEW OF PARTHENOGENESIS. 808 
If such an explanation be the true one, then it would appear that 
the difference between sexual and asexual reproduction is not so 
great as is generally supposed. 
ROTIFERS. 
The phenomena of development are very complicated in the 
Rotifers. In most cases the males differ from the females in 
being smaller and in the absence of an alimentary canal. The 
eggs are of two kinds, the same difference being seen here be- 
tween summer and winter eggs as in Aphids. Cohn (1856-58) 
first worked out the development of this group and found that the 
winter eggs are fertilized, while the summer eggs are not. Huxley 
(1857) looked on these summer eggs as sexless buds, but the work of 
Joliet (1883), Plate (1884-85) and Maupas (1889-90) established 
this as true parthenogenesis. Here, asin Aphids and Daphnids, the 
males appear at the beginning of an unfavorable period in the life 
cycle. 
Under the subject of the Maturation of Parthenogenetic Eggs the 
work on Rotifers is mentioned, and the results there recorded are 
the most interesting features in connection with the phenomenon of 
parthenogenesis in the group. The principal point of interest is 
that the male and female eggs behave differently during their 
maturation, although eggs of both sexes have the power of develop- 
ment without fertilization. 
Lauterborn (1898) found that the Rotifers could be classified into 
three groups as follows: (1) Species found all the year around ; 
(2) Species found in summer, and (3) Species found in winter. 
In the summer and winter species the fertilized and yolk-laden 
eggs appear after-a long series of parthenogenetic generations ; they 
are monocyclic. In the species found during all seasons of the year 
the appearance of the males and the consequent fertilized eggs may 
occur twice or more times during the year; they are polycyclic. 
Probably some species are acyclic; that is parthenogenetic forms 
can be produced indefinitely and ‘‘ winter’’ eggs are unknown. 
The determination of the appearance of males in Rotifers has been 
variously explained, the amounts of heat (Maupas) and nutrition 
(Nussbaum) being often considered as the causes. Lauterborn con- 
cludes that such external causes do not fully explain this but that 
some internal factor is the principal cause. The cyclic appearance 
