BILATERALITY OF THE PIGEON's EGG 281 



bryo and the whole ovum in the incubated egg (diagram II B 

 and F) . It is difficult to convince one's self that this relation is 

 of general occurrence, for there are many technical difficulties 

 involved. No confidence can be placed in preserved material and 

 it is not easy to study the fresh oocytes from all angles under the 

 immersion lens; the difference between the two diameters of the 

 germinal vesicle is rarely more than one micron, and it is not 

 apparent except in direct polar views. Still it was possible to 

 observe some oocytes in this way, and in one ovary, where the 

 conditions were especially favorable, it appeared that the angle 

 between the shorter axis of the germinal vesicle and the long axis 

 of the oocyte was relatively constant. This matter is discussed in 

 Section VI, page 299, where it is shown that the same holds true 

 for the angle between the embryonic and chalazal axes. The 

 long axis is definitely related to the embryonic axis of symmetry 

 and it has been shown that it marks the axis of bilaterality of the 

 primordial follicle. If we look upon the elliptical shape of the 

 germinal vesicle as an expression of bilaterality also, we have a 

 basis here in this nucleo-ooplasmic relation of the remarkable 

 relation between the embryo and the ovum as a whole in the incu- 

 bated egg. 



To sum up : Two axes of symmetry can be distinguished in the 

 primordial follicle; (1) The polar axis, which is marked by the 

 eccentric germinal vesicle, the yolk nucleus and the spherule cap. 

 (2) The axis of bilaterality defined by the long axis of the oocyte 

 with the germinal vesicle nearer one end of it. The relation of 

 this bilaterality of the oocyte as a whole to the bilaterality of the 

 embryo is traced in the following sections. (3) There is some 

 evidence to justify the interpretation that at this stage the ger- 

 minal vesicle has an axis of bilateral symmetry which bears the 

 same relation to the long axis of the ovum as later the embryonic 

 axis bears to this same long axis. 



B. The second growth period: {0.09 to O.If. min.) 



Sonnenbrodt ('08) has emphasized the fact that the primordial 

 follicles of the adult ovary are in a quiescent state; indeed the 



