298 KANSAS UNIVERSITY SCIENCE BULLETIN. 



of these two zones, as in the crescent-shaped one first described. 

 A definite wall separates the inner from the outer layer. 



There is one feature of this zone that gives us, perhaps, the 

 safest clue to its origin and composition. Plate XIV, figure 22, 

 shows a section through the long axis of the egg and egg-stalk, 

 and we see the inner zone extending into the epithelial cells of 

 the stem, or, more according to my belief, rising from these epi- 

 thelial cells. Now, this zone must be the food supply that is 

 being constantly brought in by these nurse-cells in the egg- 

 stalk, and deposited in the cytoplasmic network so compactly 

 that the latter structure is made invisible. In the neighbor- 

 hood of the vitelline body the mass stains more densely, show- 

 ing a gathering of the metaplasm especially at this point. 



The vitelline zone of Balbiani, or the dense cloud which col- 

 lects about the vitelline body, and is especially prominent in 

 the later stages of the follicular egg, has its origin also from 

 the food material brought into the oocyte by the nurse-cells, 

 and is probably but a condensed form of the larger zone so con- 

 spicuous in the early eggs. 



e. The Eggr-stalk. 



The above leads to a discussion of the function of those cells 

 in the stalk of the follicular egg. AVe saw how the young 

 oocyte lay first among a group of growing oocytes within the 

 wall of the ovary. (Fig. 1, e, pi. XII.) For a time all remained 

 equal in size ; then, from some advantage which one or two 

 gained over the others, they developed much more rapidly and 

 completely outgrew them. As these increase in size they 

 crowd through those around them, press up through the epi- 

 thelial layer, and even separate the muscular covering, to pass 

 through. The egg upon leaving the epithelial layer of cells 

 comes in contact with the peritoneal membrane, which it can- 

 not pierce ; but the membrane, being flexible, is pressed before 

 it through the muscular wall and there holds the egg in place. 

 The epithelial cells, having been displaced, now move back and 

 fill all space made by the departing oocyte, shoving close 

 against the lower wall of that cell. The arrangement of the 

 cells would naturally be altered and they would no longer be so 

 closely packed together, thus explaining the definite outline of 

 the stem or yolk-stalk. (Fig. 1, pi. XII.) 



