BILATERALITY OF THE PIGEON's EGG 309 



taken place, the antero-posterior axis is expressed by the oval form 

 of the blastodisc and by granule movements in periblast and 

 segmental disc. The most characteristic of the granule con- 

 figurations is a crescent formed about the posterior side of the 

 segmental disc by certain granules ('the wedge') which are 

 differentiated very early in the history of the blastodisc. These 

 granules are deutoplasmic in character and their arrangement 

 with reference to the bilaterality of the embryo is due to activi- 

 ties within the living substratum, that is, the ground substance. 

 The essential feature of the relation between the embryo and 

 the long axis of the ovum is the fact that when that end of the 

 ovum which is predetermined in the ovary to pass down the 

 oviduct first, is held to the right, the head of the embryo is 

 directed away form the observer. The actual angle between 

 embryonic and long axes is subject to much greater variation 

 than is generally supposed, but it is relatively constant for the 

 eggs laid by a given bird. 



C. Conclusions 



* 



If the evidence here offered that the structure of the primordial 

 follicle determines the end of the egg which shall pass down the 

 oviduct first, and that this end is definitely related to the embry- 

 onic axis of symmetry be accepted, the conclusion is warranted 

 that the structure of the primordial follicle is a manifestation 

 of the bilateral organization of the oocyte. In other words, the 

 antero-posterior axis of the pigeon is defined at least as early as 

 the stage of the primordial follicle. 



The conclusion that the relation between the embryonic axis 

 and the long axis of the entire egg is an expression of the bilat- 

 eral organization of the ooplasm is also supported by the fact 

 that this relation is much more constant for the eggs of a given 

 bird, than for eggs obtained from different birds. 



An explanation of the relation between the long axis of the 

 entire ovum and the axis of the embryo is suggested by a corre- 

 sponding relation found in many young oocytes between this 

 same long axis and the axis of the germinal vesicle, especially 



JOURNAL OF MORPHOLOGY, VOL. 23, NO. 2 



