926 



GENERAL METABOLISM 



[PT. Ill 



be well understood that Fig. 252, as far as absolute values go, is 

 meaningless: it would represent the absorption of embryos 50 per 

 cent, less rich in water than they really are: but it demonstrates 

 more clearly the relationship of fat and protein. In the last stages 

 of development, "it is as if the protein were displaced by fat", says 

 Murray, speaking of fat storage, and this is just what Riddle found 

 in his studies of the yolk in pigeons' eggs. At the end of development 

 there was a marked preferential absorption of fat. Thus from the 

 I St to the 6th day very Httle fat is being absorbed, but a great deal 

 of protein, from the 6th to the 12th day exactly the reverse holds, 

 from the 12th to the 17th day the protein again takes the prominence, 

 while after that time the fat once more overcomes the protein. The 

 rhythms of the two absorption curves are entirely distinct. 



At first sight, it would not 

 nificance of this. One way of 

 expressing it would be to say 

 that the cells of the blastodermal 

 blood-vessel walls which collect 

 the nourishment from the yolk 

 and the white have periods at the 

 ends and beginnings of which the 

 properties of their membranes 

 alter. At one time they will admit 

 fat-soluble substances in pre- 

 dominance, at another time 

 water-soluble substances. Putin 

 this way, the phenomenon im- 



seem easy to estimate the sig- 



Oays •— > 5 



Fig. 252. The reason for the ungraduated 

 ordinate is explained in the text. 



mediately reminds us of the state "of affairs seen by many observers 

 in the egg of the sea-urchin where there are periods of permeability 

 to water-soluble substances, and other periods of permeability to 

 fat-soluble substances. At one point the resistance to alcohol- 

 chloroform-ether cytolysis is remarkably high, at another it is ex- 

 tremely low. A similar curve can be prepared for a water-soluble 

 substance, such as potassium cyanide, and its summits are seen 

 to correspond to the troughs of the previous one. That rhythms of 

 permeability to fat-soluble and water-soluble substances should be 

 present in the single developing egg-cell of the lower animals, and 

 should then appear again in the complicated avian organism 

 during its ontogenesis is certainly possible, and, if real, of considerable 



