Centrifugal Force upon Beetles' Eggs 543 



one hour a very distinct zone may be recognized (C.B. 4, d, Fig. 

 10). The surface of the egg in this region is in every case wrinkled 

 and folded as though the volume had decreased at this end and the 

 firm layer of "Keimhautblastem" had become pulled in (C.B. 

 4, d, Fig. 10; C.B. 4, /, Fig. 13; C.B. 10, c, Fig. 14; C.B. 5, a. 

 Fig. 19). This may, however, be due to poor fixation, as these 

 folds are not visible in the eggs before they are killed. 



The vesicular zone is present as such for some time after the 

 eggs are taken from the centrifugal machine. It is not visible in 

 sections through eggs C.B. 2, c, and C.B. 5, r, which carry normal 

 embryos, but is present in C.B. 9, d (Fig. 16), which has produced a 

 shapeless mass of tissue at the inner (anterior) end. This would 

 indicate that the material of which this region is composed is 

 required for normal development. However, I do not believe that 

 this is established by the few cases observed. 



The yolk zone. A very slight amount of centrifugal force is 

 necessary to cause a noticeable disturbance in the large central 

 yolk mass. The largest yolk spheres, as shown in C.B. 4, ^, 

 are thrown to the outer heavy end within fifteen minutes after the 

 egg is centrifuged. A more marked distribution of yolk globules 

 results from a longer application of centrifugal force. A redis- 

 tribution takes place very quickly after the eggs are removed from 

 the machine; this is shown distinctly in sections through eggs 

 C.B. 2, f, and C.B. 5, c. No redistribution took place in eggs 

 revolved at a slow rate of speed for a long period; the yolk remained 

 at the heavy end in these cases and the embryos, failing to grow 

 around it, became dwarfed as shown in Figs. 21, 22 and 23. The 

 yolk has been shown to be the densest substance in the eggs of 

 other animals; for example, in the frog's egg the white yolk is the 

 heaviest material, as Born ('85) proved by sectioning those that 

 had been rotated. 



The cytoplasm. The peripheral layer of cytoplasm is lighter 

 than the gray cap material or the yolk; continued application of 

 centrifugal force causes it to rise to the inner end of the egg (C.B. 

 4, d, Fig. 10), where it becomes part of the vesicular zone. The 

 cytoplasm filling the interdeutoplasmic spaces also accumulates in 

 this region. 



