112 GERM-CELL CYCLE IN ANIMALS 
experimental conditions without killing them or 
stopping their progressive development; and they 
can be killed, fixed, sectioned, and stained with 
comparative ease. Furthermore, the eggs of these 
beetles possess a well-defined pole-disc, and the 
primordial germ cells which arise even before the 
blastoderm is formed are easily distinguishable 
from the somatic cells and thus can be traced from 
the time of their appearance until they become ma- 
ture eggs and spermatozoa. 
The ova of insects have long been considered 
among the most highly organized of all animal 
eggs. That they are definitely oriented while still 
within the ovary was expressed by Hallez (1886) in 
his ‘‘Law of the Orientation of Insect Embryos” 
as follows: ‘“‘The cell possesses the same orientation 
as the maternal organism that produces it; it has a 
cephalic pole and a caudal pole, a right side and a 
left side, a dorsal surface and a ventral surface; 
and these different surfaces of the egg-cell coincide 
to the corresponding surfaces of the embryo.” The 
orientation of an ovarian egg is indicated in Fig. 35, 
and here also is shown the position and surfaces of 
the egg at the time of deposition. When the egg is 
laid the beetle clings to the under surface of a leaf, 
and with a drop of viscid substance from the acces- 
sory glands of the reproductive organs, fastens the 
egg by its posterior end (p) tothe leaf; then with the 
tip of the abdomen the egg is pushed back through 
the are indicated by the dotted line. It is a simple 
matter to determine the various surfaces of eggs 
