THE EGG 29 



tened against the vitelline membrane, the inner surface re- 

 maining convex (Fig. 8). The point on the surface to which 

 the germinal vesicle migrates is situated away from the surface 

 of the ovary, and thus in the position of the pedicle of the 

 follicle, when the latter projects from the surface of the ovary 

 (Fig. 7). . 



The formation of the yolk has not received the attention that 

 the subject deserves; and it is possible to give only a very general 

 outline. While the nucleus is still in the center of the egg a 

 very dense deposit of extremely fine granules is formed around 

 it, and gradually extends out towards the periphery of the cell, 

 but does not involve the peripheral layer of protoplasm, which 

 is slightly thicker at the innermost side of the follicle correspond- 

 ing to the stalk. When the ovum has reached a size of approx- 

 imately 0.66 mm, the nucleus moves tow^ards the thickening 

 of the peripheral layer and enters it, lying very close to the vitel- 

 line membrane. 



The very finely granular central aggregation of yolk-granules 

 represents the primordium of the latebra or central mass of the 

 white yolk. After the nucleus has reached the periphery, or 

 while it is still on its way, the yellow yolk begins to be formed 

 by the peripheral layer of protoplasm. Small yolk-granules 

 arise near the inner margin of the peripheral layer of protoplasm 

 and increase in size; each becomes enclosed in a vacuole which 

 grows to a considerable size, so that the accumulation of vacu- 

 oles on the inner surface of the peripheral protoplasm soon 

 produces a kind of emulsion; this appears in section like a 

 reticulum, the spaces of which are the sectioned vacuoles, and 

 the strands the remains of the protoplasm in w^hich the 

 vacuoles are embedded (Fig. 7). This layer lies between the 

 peripheral unmodified protoplasm and the white yolk. A sim- 

 ilar process is going on at the same time in the central pri- 

 mordium of the white yolk, but the vacuoles and the granules 

 are smaller, and the contrast between the white and yellow yolk 

 is obvious (Fig. 7). 



Successive layers of yellow yolk are deposited around the 

 central mass of white yolk by the activity of the peripheral layer 

 of protoplasm. Thus one can understand in general how the 

 concentric arrangement of the yellow yolk is produced. But the 

 alternation of layers as seen in a section of the hardened yolk 



