Germ Cells and early Embryology of certain Aphids. 621 
potentially equal im their early history in the formation of the 
follicles, but become differentiated in their early embryonic develop- 
ment. 
The differentiation of a common group of germ cells is well 
marked in the aphids in the formation of the reproductive organs. 
The ovarian glands and nutritive string are well developed before 
the ova enter the end chamber. The condition of the ova at the 
beginning of the growth period is shown in Fig. 118. The chromatin 
granules are collected into larger masses. Fig. 119 shows the con- 
dition of the germinal vesicle in the center of the egg. The nuclear 
membrane is not very distinct. As the germinal vesicle moves to 
the periphery the chromatin collects into six distinct groups (Fig. 120). 
The nuclear area is very clear, while the cytoplasm is more granular 
and vacuolated. The wall of the germinal vesicle now breaks 
down and the nucleoplasm fuses with the cytoplasm. The chromo- 
somes become less granular and stain very dark (Fig. 121). The 
achromatic activities are not well marked, but the chromosomes 
become arranged in an equatorial plate (Fig.'122) and divide equally, 
forming a single polar body (Fig. 123—123). Six chromosomes, the 
somatic number, are found in the polar body and cleavage nucleus. 
There is no reduction in the formation of the polar body. The 
polar body does not pass out of the egg, but remains near the 
periphery in the cytoplasm. It stains as a dense homogeneous mass 
and persists after cleavage has begun, but finally disorganizes. The 
cleavage nucleus passes from the periphery to the center of the 
egg after maturation, where it enters into a short resting stage. 
The maturation of the parthenogenetic eggs is very interesting 
on the account of its bearing on the theory of fertilization by the 
second polar body and the individuality of the chromosomes. The 
six chromosomes, four large and two small, are constant throughout 
parthenogenetic reproduction and divide equally in the formation of 
the single polar body. The number of parthenogenetic generations 
does not affect the usual course in the maturation of the fertilized 
eggs. The idea that the second polar body determines the male sex 
has no significance in the parthenogenetic eggs of the aphids. 
Minor, 1877, suggested that parthenogenesis may be due to the 
failure to form polar bodies, i. e, the entire chromatin remained 
within the egg, — hermaphroditic, — and is capable of further 
development without the addition of chromatin from the male cell. 
BLocHMANN 1888, found in aphids that the eggs which developed 
