Germ Cells and early Embryology of certain Aphids. 617 
History of the germ cells. 
1. Spermatogenesis. 
The history and formation of the germ cells of the successive 
generations is one of the most striking features in the development 
of the aphids. They are the first cells differentiated and are 
recognizable when the blastoderm begins to invaginate at the 
posterior pole of the egg. The early stages in the formation of the 
reproductive organs in the male and female producing forms are 
indistinguishable. 
The adult testes of Melanoxanthus salicis are paired, each part 
consisting of five more or less radially arranged lobes. The blind 
end of each lobe is larger than the proximal end, which tapers 
down and becomes more tubular. The five lobes unite at a common 
point and form the vas deferens, which meets its fellow on the 
opposite side. Each lobe is composed of a number of cysts. In Pl. 51, 
Fig. 45, the longitudinal section of a single lobe shows eight cysts, 
containing sex cells in different stages of development ranging from 
the growth period to the nearly completely developed sperm. The 
cells of any individual cyst vary slightly in their stages of develop- 
ment. 
The cells of the last spermatagonial division are composed of 
a clear homogeneous mass of naked protoplasm. The nucleoplasm 
is clearer and less granular than the cytoplasm. At the beginning 
of the last spermatogonial division (Pl. 53, Fig. 62—63), the chromatin 
collects at definite points on the spireme (Fig. 64), and forms six 
distinct chromosomes, four large and two small (Fig. 65—67). This 
difference in size of chromosomes is a constant feature throughout 
spermatogenesis. The chromosomes arrange themselves in an equa- 
torial plate (Fig. 68). The different phases in the last spermato- 
gonial division are shown in Fig. 69—73. The division of the 
nucleus and cytoplasm is not always simultaneous. In some instances 
the daughter nuclei are in late telophase before the cytoplasm begins 
to divide (Fig. 73). The six chromosomes are distinct at the close 
of the last spermatogonial division, but immediately at the beginning 
of the growth period they lose their individuality, assume a granular 
condition (Fig. 75), and collect in a single homogeneous mass within 
the center of the nucleus. The chromatin remains in this condition 
during the growth period (Fig. 75—76). In the early prophase of 
Zool. Jahrb. XXIV. Abt. f. Anat. 41 
