542 R. tV. Hegner 



guishable: (i) a bright-orange light zone at the inner end (the 

 vesicular zone, Fig. 20, v. z), (2) a comparatively large central 

 mass composed of yolk globules which are largest at the outer 

 heavy end, gradually becoming smaller until indistinguishable from 

 cytoplasm at the inner end, and (3) a colorless layer (the gray cap, 

 Fig. 20, g. c) at the extreme heavy end. These three zones are 

 produced when the eggs are oriented either with their posterior 

 (C.B. 4, e) or their anterior (C.B. 3, b) ends toward the axis of 

 rotation. When placed with their sides toward the center only 

 two layers are induced, the vesicular zone and the yolk zone. 

 Three layers may be obtained in fresh eggs (C.B. 4, e) in eggs 

 which have reached a late cleavage stage (C.B. 2, b, Fig. 18) and 

 in eggs which are covered by a blastoderm (C.B. 5, a. Fig. 19). 



The gray cap. The material of the gray cap is the heaviest of 

 the egg contents. It is composed of very fine granules whose 

 positions before being driven to the heavy end of the eggs could not 

 be determined A fresh egg when centrifuged for one hour does 

 not exhibit this layer (C.B. 4, d, Fig. 10). At the end of two hours, 

 however, a distinct gray cap is present (C.B. 4, e). Eggs in late 

 cleavage stages require a lesser amount of centrifugal force in order 

 to produce this structure (C.B. 2, b. Fig. 18). We conclude from 

 this that either the gray cap material is liberated during develop- 

 ment and the egg fourteen hours old (C.B. 2, h) contains a greater 

 quantity of it, or else some condition of the yolk mass at this age 

 allows it to pass more rapidly toward the heavier end. Longitu- 

 dinal sections through egg C.B. 2, c (Fig. 7) show that although 

 the embryo has developed normally the material of the gray cap 

 is still at the heavy end where it was driven by the centrifugal force. 

 A like condition also exists in a slightly younger egg (C.B. 5, c). 

 It is evident that the gray cap substance is not necessary for the 

 normal development of the embryo. 



The vesicular zone. The light fats which probably produce the 

 vesicular zone at the inner end of the egg collect very quickly under 

 the influence of centrifugal force. An egg centrifuged for only 

 fifteen minutes (C.B. 4, b) has a small number of vesicular spaces 

 near the pole-disc. Continued application of centrifugal force 

 results in a g-reater number of these vesicles until at the end of 



