SECT. I] PHYSICO-CHEMICAL SYSTEM 319 



and this is duly reflected in the chemical composition by the moderate 

 water-content, the high proportion of protein which is yet only 

 double that of the fat. The case of the dogfish is again different, for 

 there the egg is rich in yolk and the cleavage is meroblastic; thus 

 the water is rather low, the fat rather high, the nitrogen very high, 

 and the ash and phosphorus moderate. 



But these conclusions of Wetzel's, interesting though they are, can- 

 not really be assessed until a great deal more comparative work has 

 been done. They must rather be taken to represent the kind of cor- 

 relation we may hope for in the future. However, one of Wetzel's 

 generalisations may be accepted, if with some reserve. He pointed 

 out that the fat-content of eggs showed great variations, rising from 

 12 per cent, of the dry weight of the Sepia ^gg to 66 per cent, of the 

 dry weight of the (yolk of the) hen's tgg. Again, the nitrogen 

 gave very variable results, rising from 5-3 per cent, of the dry 

 weight in the (yolk of the) hen's ^gg to 6-9 per cent, in the egg 

 of the grass-snake, 1 2 per cent, in the egg of the dogfish, and even 

 in the case of the cod 14 per cent. On the other hand, the phosphorus- 

 content varied only between the (outside) limits of 2 • i per cent, for 

 the sea-urchin tgg and 3-6 per cent, for that of the grass-snake. 

 Wetzel, therefore, suggested that a distinction might be made, at any 

 rate, roughly, between those constituents of the egg which may serve 

 as sources of energy for the growing embryo, and those which in 

 no circumstances do so. Protein, fat, and carbohydrate would come 

 in the former class; phosphorus (for nucleoprotein) and cholesterol, 

 for example, would come in the latter class. The former would show 

 great variations among eggs of different species, the latter would not. 

 He thus supposed that one might be able to deduce, as it were, the 

 constitution of any given egg, if one knew what substances, and in 

 what proportions, were used by the embryo as combustible material 

 during its development, as well as the constitution of the newly born 

 or hatched organism. 



From this standpoint Wetzel distinguished four types of substances 

 in the unincubated egg : ( i ) material for the embryo to burn during 

 the course of its development, (2) constituents of the finished proto- 

 plasm of the embryo, (3) constituents of the finished embryo, but 

 not for incorporation into the protoplasm itself, but into the para- 

 plasm (in Le Breton's terminology), (4) the protoplasm of the original 

 egg-cell. No aspect of chemical embryology needs attention more 



