282 THE EVOLUTION THEORY 



111 these Algfe we meet, for the first time, with a special organ in 

 whicli the ova arise. In animals this is much more generally the 

 case, and from sponges upwards there are always quite definite parts 

 and tissues of the body wliich are alone able to develop eggs, and 

 tliese are usually well-defined organs of special structure, the ovaries. 

 Similarly, in male animals the spermatozoa arise in special places, and 

 usually' in special organs, the spermaries or testes. 



Animal ova often consist of more than the simple cell-body, the 

 protoplasm and its nucleus ; the}^ almost always contain in the cell- 

 body a so-called 'Deutoplasm,' as Van Beneden has fittingly named the 

 yolk-substance. This consists of fats, carbohydrates, or albuminoids, 

 which often lie in the cell-body in the form of spherules, flakes, or 

 grains— a nutritive material that is often surrounded and enclosed 

 by a small quantity of living matter or formative protoplasm. Apart 

 from these stores of yolk it would be impossible for a young animal 

 to develop from the ovum of a snake or a bird, for such highly 

 differentiated animals could not be formed from an egg of microscopic 

 dimensions if this remained without some supply of food from outside 

 of itself during the period of development. There is obviously need 

 for a considerable amount of building material, so that all the organs 

 and parts, which are composed of thousands and millions of cells, may 

 be developed. 



Thus the size of the animal-ovum depends essentially on the 

 quantity of yolk that has to be supplied to the egg, and this 

 depends in the main on whether the egg is still drawing nourishment 

 from the mother during the development of the young animal. There- 

 fore, as a general rule, eggs which are laid, and are surrounded and 

 protected by a shell, are usually much larger than the eggs of animals 

 which go through their development within the body of the mother. 

 The best known illustration of this ^proposition is offered by mammals 

 and birds, animals of equally high organization and comparable in 

 bodily size. While the eggs of birds may be as nmch as 15 centi- 

 metres in length, and may weigh i^ kilogrammes, those of most 

 mammals remain microscopically minute, and scarcely exceed a length 

 of 0-3 millimetres. The same principle is often illustrated within one 

 and the same small group of animals, and even in the same species. 

 Here, again, the Daphnids or water-fleas may serve as an example. 



Among these there are two kinds of eggs, summer and winter 

 eggs, of which the former go through their development into a J^oung 

 animal within a brood-cavity on the back of the female, while the others 

 are liberated into the water, and are surrounded by a hard shell. The 

 summer eggs receive more or less nourishment from the mother by 



