I204 METABOLISM OF LIPOIDS, STEROLS, [pt. iii 



yolk, this is probably the case, so that the presence of the membranes 

 in Plimmer & Scott's "remainders" will perhaps not wholly account 

 for the rise in nucleoprotein phosphorus outside the embryo. The 

 ether-soluble phosphorus outside the embryo behaves in a manner 

 very similar to that seen on the curves for the whole egg, but inside 

 the embryo it seems to show a fall towards the end of development. 

 This may possibly be a chemical expression of cephalocaudal differ- 

 ential growth (see p. 583) ; in other words, the lecithin phosphorus 

 in per cent, of the total phosphorus in the embryo may decHne owing 

 to the fact that the brain and central nervous system of the embryo 

 are declining in percentage of the total weight. Plimmer & Scott's 

 figures for the phosphorus distribution in the embryo were not very 

 numerous, and it would be exceedingly desirable to extend them 

 in a backward direction, so that we might know the phosphorus 

 distribution in an embryo 3 or 4 days old. It might be predicted 

 that the ether-soluble phosphorus would be high and everything 

 else low, except, perhaps, the water-soluble organic phosphorus. 

 In this connection the results of Marza are of interest. Using the 

 histochemical method of Romieu, which is said to indicate the 

 presence of lecithin, he observed a distinct lag between the for- 

 mation of the neural portions of the early chick embryo and the 

 appearance of lecithin. A gradient was also perceptible, the amount 

 of lecithin at a given time decreasing from cephalic to caudal end. 

 This would suggest that the morphological pattern of the early 

 neural elements is sketched out, as it were, in a protein-carbohydrate 

 medium, while the lipoids, so essential a constituent of the fully 

 developed central nervous system, are brought in later. All this 

 fits in remarkably with what we know about the composition of 

 white and yellow yolk (see p. 286). Another point of some interest 

 arises out of Fig. 373, where the vitellin phosphorus of the whole 

 egg seems almost as abundant on the 13th or 14th day of develop- 

 ment as it was on the 3rd or 4th. This is another reflection of the 

 fact already so often seen from different angles, namely, that the yolk 

 is "a remoter and more deferred entertainment than the white". 



The water-soluble organic phosphorus is perhaps the most inter- 

 esting of all the fractions. On both the graphs for the phosphorus 

 distribution of the whole egg, it can be seen that this fraction, be- 

 ginning at about 5 per cent., increases slowly to about 10 per cent, 

 in the middle of development, and finally falls away again to its 



