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1903. ] PHILLIPS—A REVIEW OF PARTHENOGENKSIS. dll 
form the primordial germinal cells. The first’ maturation division 
gives off a true polar body. 
Causes of Parthenogenesis.—When we consider the difference in 
behavior of various parthenogenetic eggs during maturation and the 
differences in sex relations exhibited by the various groups, together 
with the wide range of the scattered cases where such development 
occurs, it is evident that parthenogenesis has had a separate origin 
in many places in the animal scale. All that is necessary in the 
maturation of a parthenogenetic egg is that the normal number of 
chromosomes shall be retained, and this may be brought about by 
the retention of the second polar body, fertilization by the second 
polar body or perhaps by the division of the chromosomes without 
the corresponding cell division. 
In seeking for a cause for the appearance of parthenogenesis in a 
group of animals, it must be borne in mind that we are dealing with 
a phenomenon that to all practical purposes is like asexual reproduc- 
tion, in that the species is not dependent on the union of the two 
sexes for the propagation of all the individuals of the species and 
that the causes for the appearance of asexual and parthenogenetic 
reproduction are practically identical, it being merely a question as 
to which method of agamic reproduction is most readily acquired 
by a given form when the necessity for such a thing arises. And, 
too, it is probable that the cause is not the same in all cases, since 
the environments and habits of the various forms possessing this 
power are so varied. 
In the first place, parthenogenesis is generally associated with 
and probably caused by the necessity of the appearance of a great 
many individuals suddenly at a certain period of the year or of the 
life cycle. A large part of the forms exhibiting this method of 
reproduction are small short-lived animals which are represented 
during the winter or some adverse time in the life cycle by a very 
few individuals and, in order that the species may survive, are 
compelled to acquire some method of rapid agamic reproduction. 
In the case of the Aphids the necessity is for females and we find 
thelytoky evolved ; in the case of the Honey Bee the necessity is 
for males, so that the queens may not go unfertilized, and we find 
arrenotoky. 
The question of economy enters very largely into the problem and 
is, in fact, almost identical with the preceding cause. In many 
