BILATERALITY OF THE PIGEON's EGG 307 



be seen when these eggs are sectioned with reference to the gray 

 crescent. 



Reptiles. The statement of Will ('93, p. 15) that in the gecko 

 egg the long axis of the embryo is approximately perpendicular 

 to the longest axis of the entire ovum is strong evidence that 

 conditions similiar to those described here exist in that egg. The 

 early appearance of the antero-posterior axis of reptiles is sug- 

 gested through the analogy of the bird's egg, by the fact that most 

 workers on the early stages have found the blastodisc more or 

 less elliptical in shape. 



Birds. Coste ('47), Kolliker ('76), Duval (in his atlas) and 

 Patterson ('10) have figured surface views of the hen's egg in 

 precleavage and early cleavage stages. In every case the segmen- 

 tal disc, the inner periblastic zone and the periblastic ring are 

 shown and Kolliker figures a narrow outer zone about the latter. 

 With one exception these zones are drawn circular, but Patterson's 

 figure 13 shows them elliptical and the shorter axis of the ellipse 

 formed an angle of 90 degrees with the oviducal axis and coin- 

 cided therefore with the embryonic axis. It is probable that 

 in the hen's egg the bilaterality is not so clearly manifested by 

 the change in shape of the blastodisc as is the case in the pigeon's 

 egg. As has been said above, the orientation in the hen's egg 

 seems to be less constant, but the matter needs further study. 



Mammals. The definite orientation of the embryos of various 

 mammals with reference to the chorionic vesicle and the uterus 

 suggest that here too the antero-posterior axis may be traced 

 to an early stage of development. 



B. Summary 



See diagram II, p. 279 



A definite relation exists in the pigeon's egg between the axis 

 of the embryo and the long axis of the ovum. 



Both the embryonic and the long axis are present in the ovarian 

 egg, that is, the antero-posterior axis of the pigeon is predeter- 

 mined in the ovary. 



The ovarian history may be divided into four periods in each 

 of which the oocyte exhibits a characteristic organization. 



