286 THE UNFERTILISED EGG AS A [pt. iii 



In the fresh egg, as laid, the white yolk occupies a central position, 

 and is surrounded by concentric layers of yellow yolk. But as a kind 

 of cylindrical prolongation of the white yolk reaches to the surface 

 of the vitellus underneath the blastodisc or germinal spot, the white 

 yolk must be considered the first food of the embryo, and, until its 

 composition was determined, it was not possible to say what sort 

 of nutrient environment the embryo possessed in the very early days 

 of development, although the composition of the yellow yolk would 

 give this for the later period. The histological differences between 

 white and yellow yolk had been known for a long time (see Purkinje ; 

 His ; Leuckart ; Klebs ; Dursy ; Strieker ; and Virchow) but Riddle 

 and Spohn & Riddle were the first to approach the subject chemically. 

 Their figures showed that the white yolk much the more nearly 

 approximated to the contents of invertebrate eggs with holoblastic 

 cleavage, and living undifferentiated tissue generally. Instead of 45 per 

 cent, of water, the white yolk had 86 per cent., instead of 15 per cent, 

 of protein, it had only 4, and instead of 25 per cent, of fat it had 

 only 2. Thus in its water-content, it was much more like {a) egg- 

 white and {b) the young embryo itself than like ordinary yolk, while 

 instead of having twice as much fat as protein it had twice as much 

 protein as fat. These data are extremely interesting in view of the 

 facts that are known about the sources of energy made use of by the 

 embryo during its development. Although by far the greatest pro- 

 portion by weight of substance combusted during embryonic life is 

 fat, yet, in the early stages, the embryo undoubtedly gets its energy 

 preponderantly from protein and carbohydrate (see the whole of 

 Section 7). The percentage of non-nitrogenous extractives did not 

 differ much between white and yellow yolk in the experiments of 

 Spohn & Riddle, but it would be very interesting to know the 

 relative amounts of carbohydrate, and analyses to discover this should 

 certainly be done. Again, the yellow yolk contained eight times less ash 

 than the white yolk, a finding which acquires considerable significance 

 from the fact that, if the ratio inorganic substance/organic substance 

 in the embryonic body is plotted, it is seen to descend steadily 

 from the beginning of development (see Fig. 249). Moreover, as 

 Mendeleef has shown, early embryonic cells contain twice as much 

 electrolyte as those of later stages (see Section 5"8). The amount of 

 phosphatide in the yellow yolk, furthermore, was ten times that in 

 the white, a significant difference; for, as Plimmer & Scott have shown, 



