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) to the leaf ; then with the 

 tip of the abdomen the egg is pushed back through 

 the arc indicated by the dotted line. It is a simple 

 matter to determine the various surfaces of eggs 



