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1903.] PHILLIPS—A REVIEW OF PARTHENOGENESIS. 297 
the fall was enough to furnish what he designated as ‘‘ spermatic 
force ’’’ for the development of the numerous summer generations. 
«Tn the vertebrated and higher invertebrated animals only a single 
individual is propagated from each impregnated ovum. Organized 
beings might be divided into those in which the ovum is uni- 
parous and those in which it is multiparous. This is the first and 
widest or most general distinction which we have to consider in 
regard to generation, and in proportion as we may recognize its 
cause will be our insight into the true condition on which Par- 
thenogenesis depends.’’ * 
The next step in advance was made when it was discovered by Ley- 
dig that there is no observable fundamental difference between the 
ova of the viviparous and oviparous females. There is, of course, a 
great difference between the summer eggs which develop partheno- 
genetically and the winter eggs as to size and amount of yolk, but 
this is only such a difference as may be observed between the eggs 
of various species and in no way argues for a dissimilar origin. 
This, then, put the Aphid development in the same class with that 
of Solenobia, Apis and other species known to develop from unfer- 
tilized eggs ; but so firm a hold had the idea that fertilization is neces- 
sary to the development of a true egg that Huxley (1858) and Lub- 
bock (1857) gave the name ‘‘ Pseudova’’ to the eggs of the vivi- 
parous females. From this time on it has been held that the 
viviparous development was a case of true parthenogenesis. 
The Alternation of Generations and parthenogenetic development 
is further complicated by other factors. Thus in Aphids the last of 
the viviparous generations is a generation known as the sexupara, 
the parthenogenetic and viviparous descendants of which are 
winged males and wingless females. After copulation, these 
females lay the fertilized winter eggs. This cycle of develop- 
ment is still further complicated by migrations from one plant host 
to another. A winged parthenogeretic generation frequently 
appears, and then may migrate to a different plant there to repro- 
duce itself, and in a later generation return to the original host 
(Lichenstein, 1875). These generations have been distinguished 
by Blochmann (1889) as emigrants, alienocole and remigrants. 
Thus Pemphigus terebinthi (Derbes, 1872) gives rise to a wingless 
parthenogenetic generation (@), which produces another winged 
1Page 62, Joc. cit. 
PROC. AMER. PHILOS. soc. xLIt. 174. u. PRINTED D2c. 15, 1903, 
