Germ Cells and early Embryology of certain Aphids. 633 
In the other the cleavage does not take place until after the eggs 
are deposited. 
The mode and formation of the nutritive elements are of special 
interest in the development of the aphid embryo. The most prominent 
part in the early formation of the reproductive organs is the ovarian 
glands, which become differentiated before the ova enter the oviduct. 
Their function is entirely nutritive and become connected with the 
ova by means of the nutritive string. These nutritive glands are 
entirely used up and are seldom found in an individual, where all 
the embryos are completely developed. In the parthenogenetic forms 
the ovarian glands are poorly developed, while in the sexual they 
are highly specialized and play an important part in the formation 
of the primary yolk. 
The primary yolk is very scanty in the parthenogenetic ova and 
when the blastoderm of the developing embryo begins to invaginate 
the embryo is supplied with the secondary yolk, which enters the 
embryo at the posterior end of the blastoderm. In the sexual 
developing eggs the primary yolk is very abundant. The secondary 
yolk is formed in the posterior end of the sexual ego just before 
maturation. 
The relation of the secondary yolk to the invaginating blastoderm 
is quite similar in the parthenogenetic and sexual embryos and has 
the same function in both cases. 
The abundance of yolk in the sexual ovum and its scarcity in 
the parthenogenetic ovum at the beginning of embryonic development 
does not modify or change the result of the developing embryo. 
The cleavage and behavior of the cleavage nuclei in both instances 
are the same. 
There is an interesting distinction in the behavior of the 
invaginating blastoderm. In the sexual developing embryo where 
we would expect the abundance of yolk to interfere with the 
invagination, the invagination is more complete, i.e. the germ band 
before we can detect any differentiation of organs is completely 
invaginated or immersed within the yolk and separated from the 
uninvaginated part of the blastoderm which now becomes the serosa. 
The result is that we have the undifferentiated germ band entirely 
free within the yolk, which is now completely surrounded by the 
epithelium or serosa (Pl. 51, Fig. 51). On the other hand, in the 
parthenogenetic developing embryo with a scarcity of yolk the in- 
vagination is less complete, and the uninvaginated part of the blasto- 
Zool. Jahrb. XXIV. Abt. f. Anat. 42 
