BILATERALITY OF THE PIGEON's EGG 289 



the ovaiy in all possible relations to the direction of the force ot 

 gravity and as the germinal vesicle migrates peripherally many of 

 them are subjected to an increasing strain by the force of gravity, 

 since the cell is growing larger and the animal pole is constantly 

 growing specifically lighter than the vegetative pole. If the strain 

 in these oocytes were to be released by the formation of a lymph 

 space, so that they could rotate within their follicles, they would 

 naturally swing into various positions according to their locations 

 in space (fig. 7) . At this very time a structure arises which may 

 be interpreted as such a lymph space; it is the zona radiata which 

 is formed apparently from the follicular epithelium. No method 

 of showing that the rotation takes place between the follicular 

 epithelium and the vitelline membrane was found: the evidence 

 rests solely upon the fact that rotation first appears after the zona 

 radiata has been formed, and upon the absence of any other 

 structural condition that would permit rotation. The striations 

 of the zona would, according to this interpretation, be intercell- 

 ular bridges extending from the follicle cells and they would offer 

 no obstacle to the freedom of rotation. 



This freedom of rotation of the oocyte within its follicle is a 

 matter of importance, for whatever may have been the axial 

 relations in the earlier stages, those shown as typical in diagram 2 

 are eventually estabhshed through its agency. As soon as the 

 final growth period is initiated and yolk is accumulated, the fol- 

 licle becomes large enough to hang down freely into the body 

 cavity; then its highest point is the middle of the attached pole 

 where the blood vessels enter. The oocyte now orients itself 

 along lines of least resistance, the animal pole, i.e., the germinal 

 vesicle with the anlage of the blastodisc come to lie at the attached 

 pole of the follicle and the long axis of oocyte and follicle becomes 

 perpendicular to the polar axis. The laying down of yolk with 

 reference to the polar axis is controlled by the ooplasm, but the 

 follicle, in large measure, determines the long axis, though the 

 peripheral protoplasm may also play a directive role. This orien- 

 tation of the oocyte makes it possible for the original long axis 

 which determined the follicular long axis, to persist during this 

 period when the great bulk of the oocyte is made up of inert 



