EARLY DEVELOPMENT IN THE FROG. 32! 



was away from the heart and therefore arterial. The movement 

 of blood over the unpigmented hemisphere, where the movement 

 of corpuscles could be seen more clearly than over the pigmeAted 

 hemisphere, was always toward the heart and therefore venous. 

 As was noted for the injected specimens, a small per cent, of the 

 eggs showed arteries passing a little distance over the yolk. 

 But it is especially to be noted that in every case observed, the 

 greater part of the arterial blood supply was restricted to the 

 pigmented hemisphere. 



In the ovary of the frog, the polar axes of the eggs lie in every 

 possible direction with respect to gravity, so that the possibility 

 of polarity being determined in the ovary with respect to gravity 

 is at once eliminated. 



The data indicate that polarity in the frog egg arises at some 

 time during the growth stages, in response to external conditions, 

 viz., to the blood supply of the egg: that region of the oogonium 

 chancing to be most richly supplied with arterial blood being 

 destined to become, by virtue of this respiratory and nutritive 

 relation, the animal pole of the egg. Naturally, the region on 

 the surface of the egg where the capillary net work is most 

 extensive would be effective in determining polarity rather than 

 the point where the blood vessels enter the theca. 



A region of higher oxidation rate in the egg, or at any rate the 

 proximity of this region to a greater oxygen supply (arterial 

 blood) is further evidenced by the appearance of pigment a 

 melanin (Kellicott) over a limited surface of the egg, for it is 

 well known that oxygen is necessary for the formation of these 

 pigments. It is probable that the origin of polarity and the 

 appearance of pigment in a symmetrical relation to that polarity 

 are both expressions of the localized oxygen supply (or nutritive 

 supply, or both) of the egg. 



It may be noted here also that pigment appears most densely 

 in the most active regions of the egg where other evidence 

 indicates that oxidations are proceeding more rapidly than else- 

 where. In other words, the density of pigmentation seems to be 

 an expression of the rate of at least certain oxidations occurring 

 in that region. Local increase in activity results in the formation 

 or increase of the pigment and local decrease in activity results 

 in diminution of pigment. Those unpigmented cells lining the 



