T04 THE EVOLUTION OF SEX. 



^Ir Irvine, of Granton, has recently shown that fowls kept with 

 access to no carbonate, but only to other salts of lime, can still 

 form a normal shell. This still consists of carbonate of lime, 

 and is as firm as usual, demonstrating, like the same investi- 

 gator's experiments on crabs, that animals possess no little 

 power of changing one salt of hme into another. Then, 

 in the eggs of other birds, the import of the seven or 

 more pigments which produce the marvellous variety and 

 beauty comes into question. Sorby has shown that they 

 are related to the pigments of blood and bile; but what 

 they exactly mean no one yet knows. Wider still, the 

 problem arises of how this coloration is so often protective; 

 and whether Lucas is right in supposing, that the colour of 

 the surroundings can actually influence the deposition of pig- 

 ment, by acting on the ntrvous system of the mother bird. 

 Or again, there is the curious fact, that the size of the egg is 

 often much out of proportion to the size of the bird, and the 

 question arises as to how far this can be interpreted as the 

 result of the more or less anabolic and sluggish constitution. 



§ 7. Cheijiisb-y of the Egg. — Every one knows that the eggs of birds form 

 highly nutritious diet. As the egg contains nourishment for the young bird 

 for a considerable time, it must, like milk, contain all the essentials of food. 

 The results of a recent analysis of the fowl's egg may be taken as a 

 sample. 



The germinal or formative disc consists chiefly of albuminoid bodies, 

 apparently of the globulin group, plus smaller quantities of lecithin and 

 the like. The subtle protoplasm itself, it need hardly be said, defies 

 analysis. 



In the yolk there are firm fats (tripalmitin, probably plus a little 

 stearine), and a fluid oil or glyceride. Fatty acids develop during hatch- 

 ing. A relatively large quantity of lime is present, probably, for the most 

 part, as calcium albuminate. In the white of egg there are true albumins, 

 also globulins, and the quantity of peptones increases with the age of the 

 egg. During development the embryo becomes richer in mineral matters, 

 fat, and albumen, and the dry substance of the whole contents of the egg 

 diminishes considerably. 



The yolk of many different kinds of ova has been analysed, and the 

 component substances distinguished as IchtJihi (fishes), Emydiii (tortoise), 

 and the like. More important were the discoveries of cholesterin, vitellin, 

 midein, lecithin^ and, in association with the latter, neiirin. As we cannot 

 here enter into the physiological import of such substances, it is enough to 

 say that the nutritive material in ova usually consists of a mixture of com- 

 plex, unstable, and highly nutritive substances. 



§ 8. Matnratio?t of the Ovum. — When the egg-cell has 

 attained its mature size, a more or less enigmatical occurrence 

 takes place. The nucleus, hitherto generally central, moves to 



