Germ Cells and early Embryology of certain Aphids. 625 
complete reticular condition of the cytoplasm. The yolk granules 
that are deposited in the reticular spaces grow very rapidly. The 
peripheral layer of cytoplasm or periplasm is less reticular and 
does not contain any yolk granules. The nucleus or germinal vesicle 
is found in the center of the egg with a well organized nuclear 
wall. 
The reduction of chromosomes occurs during the growth period 
of the ovum. The behavior of the germinal vesicle is very interesting. 
Starting with the germinal vesicle in the center of the ovum (Fig. 15), 
the nucleoplasm becomes differentiated into two distinct areas. A 
very dense granular outer part and a clearer inner part free from 
granules which contain the chromatin. The chromatin granules 
become more distinct and form a definite spireme (Fig. 16—17). 
The germinal vesicle during the process of maturation moves from 
the center of the egg to the periphery. The spireme thread divides 
transversely and forms three distinct chromatin rods (Fig. 18—19). 
The germinal vesicle increases in volume during maturation as it 
passes to the periphery. The wall of the germinal vesicle does not 
break down until after it reaches the extreme periphery and the 
polar bodies are formed. 
The achromatic activities of mitosis are absent and the chromo- 
somes act more or less independently although they pass through 
the same conditions in division as if the true spindle was formed. 
The chromosomes divide longitudinally in the early prophase of the 
first maturation division (Fig. 20) and the first polar body is formed 
(Fig. 21). The second division probably separates univalent chromo- 
somes. The second polar body is represented in Fig. 22—23. 
After the formation of the polar bodies the chromatin of the 
pronucleus becomes granular (Fig. 24) and the wall of the germinal 
vesicle breaks down. The nucleoplasm fuses with the periplasm 
around the entire periphery of the egg, and forms the band of 
cytoplasm which becomes evenly distributed over the entire surface 
of the egg and later becomes the cytoplasm of the blastoderm cells. 
The fusing of the nucleoplasm and periplasm is represented in 
Fig. 25. The polar bodies now appear as two compact masses of 
chromatin. The pronucleus at this stage has a granular appearance 
and is scarcely distinguishable. In a tangential section of a little 
later stage shown in Fig. 26 the polar bodies are still plainly visible 
and the pronucleus has slightly increased in size and the three 
chromosomes are in the vesicular condition. This section gives a 
