98 THE EVOLUTION OF SEX. 



porous limy shell. There arises the difficult problem of the origin of 

 the shell, in regard to which it is to be noted that Mr. Irvine, of 

 Granton, has recently shown that fowls kept with access to no car- 

 bonate, 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, demon- 

 strating, like the same investigator's experiments on crabs, that animals 

 possess no little power of changing one salt of lime into another. 

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

 pigments which produce the marvelous 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 color of the surround- 

 ings can actually influence the deposition of pigment, by acting on the 

 nervous 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 inter- 

 preted as the result of the more or less anabolic and sluggish con- 

 stitution. 



VII. — Chemistry 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 yelk there are firm fats (tripalmitin, probably plus a little stearine), aed 

 a fluid oil or glyceride. Fatty acids develop during hatching. A relatively 

 large quantity of lime is present, probably, for the most part, as calcium 

 albuminate. In the white of eggs there are true albumens, also globulins, and 

 the quantity of peptones increases with the age of the egg. During develop- 

 ment the embryo becomes richer in mineral matters, fat, and albumen, and the 

 dry substance of the whole contents of the egg diminishes considerably. 



The yelk of many different kinds of ova has been analyzed, and the com- 

 ponent substances distinguished as Ichthin (fishes), Emydin (tortoise), and the 

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

 lecithin, and, in association with the latter, neurin. As we can not 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 complex, unstable, and 

 highly nutritive substances. 



VIII. Maturation 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 the pole, 



alters considerably in its structure, and divides. A minute cell, with 



half of the nucleus, and a small amount of protoplasm, is given off. 



Not long after, the nucleus remaining within the ovum repeats the 



