BILATERALITY OF THE PIGEON's EGG 283 



them, marks the beginning of the period of differentiation (see 

 p. 286). 



2. The origin of the stigma and the follicular blood supply. The 

 characteristic 'stigma' of the ovarian folhcles in the bird has long 

 been known, but so far as I am aware nothing has been pubUshed 

 concerning its origin, relations or its exact role in ovulation. Since 

 the long axis of the oocyte has not been understood either, no one 

 has noted that the stigma is almost invariably in the long axis. 



Mechanical factors seem to play an important part in the dif- 

 ferentiation of the stigma and of the closely correlated blood supply 

 of the follicle ; at the same time both are definitely related to the 

 bilateral organization of the oocyte. 



The stigma does not necessarily arise at a definite stage in the 

 development of the oocyte; it is present in some primordial fol- 

 licles of 50 IX and occasionally follicles of 500 /x are found imbedded 

 in the stroma which show no trace of it. The reason for this is 

 that the essential feature of stigma formation is the intimate 

 association of the follicular epithehum of the oocyte with the 

 peritoneal ('germinal') epithelium, and this association is con- 

 ditioned by the oocyte's reaching the surface of the ovary. How 

 early in the embryonic history this association of epithelia may 

 take place has yet to be studied, but it is known that neither 

 oocytes nor folhcle cells develop directly from the superficial 

 layer of the ovary, 'les cellules indiferentes superficielles' of D'Hol- 

 lander ('04), so it can hardly come about until the folhcle is formed 

 and the oocyte begins to grow. It has already been said that the 

 oocytes are so oriented in the stroma that the long axis is approx- 

 imately parallel to the surface of the ovary. Accordingly, when 

 the follicle reaches the surface, the fusion first occurs in the great 

 circle of the long axis and an elongate area of fusion is soon 

 formed, the longer diameter of which coincides with the long axis 

 of the oocyte. This appears in figures 3 and 4 where the region 

 free from capillaries indicates roughly the area of stigma fusion. 

 Coincident with the apposition of the two epithelia there is a 

 flattening out of the germinal epithehum. A correlation is estab- 

 lished here, between the long axis, itself a manifestation of the 

 egg organization and the other elements of the ovarian folhcle, 



