BILATERALITY OF THE PIGEON's EGG 285 



delicate continuous layer, the theca folliculi, closely applied to the 

 follicular epithelium, and an 'outer loose theca vasculosa which 

 develops hand in hand with the capillary plexus. The theca 

 vasculosa is absent, naturally, in the region of stigma fusion, for 

 here the delicate outgrowths of the capillaries do not penetrate. 

 Figures 5, 7, and 34 show how sharply the stigma is defined in 

 injected material. 



The character and development of the blood vessels in the 

 theca vasculosa is illustrated in figures 3 to 7 and 31 to 34, all of 

 which were taken from whole oocytes cleared in creasote. During 

 the second growth period the vascular network comes to invest 

 the entire oocyte except the stigma which extends along the free 

 pole and up toward the attached pole at either end of the long 

 axis, thus dividing the vascular theca into halves which are 

 symmetrically placed with reference to the long axis (figs. 4 A 

 and B, and 5.) So it comes about that the stigma, which dif- 

 ferentiated with reference to the long axis, in turn plays a part 

 in determining the bilateral arrangement of the blood supply. 

 During the succeeding period of differentiation the bilaterality 

 of the blood supply of the follicle is accentuated by the appearance 

 of a median artery and vein on either side, which lie, roughly in 

 the plane of the polar axis, i.e., perpendicular to the long axis 

 (figs. 6 and 7, and the intermediate stages, figs. 31 to 34). The 

 arrangement of the blood vessels, illustrated in figures 7 and 33, 

 is typical of that found in the subsequent stages, although an outer 

 set of vessels, which develops during the final growth period, 

 makes the bilateral arrangement less obvious, as the larger oocyte 

 in figure 37 shows. 



The bilaterality of the blood supply is of interest because it 

 plays a part in the preservation of the bilaterality of the ovum 

 as a whole during the final growth period, when the living ooplasm 

 is confined to the region of the blastodisc and to a delicate film 

 of protoplasm over the periphery of the oocyte. 



To summarize: The long axis appears as an expression of the 

 bilateral organization of the oocyte. It determines the position 

 of the stigma and together they determine the bilaterality of 

 the blood suppl}^ which helps to insure a bilateral deposition of 



