THE EVOLUTION THEORY 117 



nucleus (fig. 31). That the egg of a mammal, which develops in 

 the shelter of the maternal body and is fed directly by it, must 

 differ from a bird's egg, which is laid at an early stage and 

 exposed to unfavourable external influences, needs no specific 

 mention. 



The first important feature of the egg of a fowl is its 

 enormous size compared with the egg-cells of most other animals 

 of the size of a fowl. The ovum of Man measures but the fraction 

 of a millimetre, and other animal ova are still smaller. But when 

 we examine the structure of a bird's egg we see that its size is due 

 to a secondary circumstance, the enormous accumulation of dead 

 food material, the yellow and white yolk and the albumen. As 

 the growing bird-embryo is dependent entirely upon itself and 

 cannot be fed by its mother, it becomes necessary that it is 

 supplied with a sufficient quantity of food. That this is the real 

 cause of the difference in size is further proved by the fact that 

 the only mammals which are not viviparous, the Duckmole 

 (Ornithorhynchus) and the Porcupine Ant-eater (Echidna), quaint 

 inhabitants of the Australian Archipelago, betraying in the 

 organization of their bodies their descent from the reptiles, lay 

 large eggs rich in yolk. 



The shell of the bird-egg is not a part of the germ-cell but 

 an organ of protection secreted by the maternal oviduct. The 

 real cell is the minute light spot which observation reveals in 

 the yolk. This consists, like the egg-cells of other animals, of 

 protoplasm and the nucleus. 



This remarkable uniformity does not only apply to the 

 structure of the ova, but extends also to the developmental 

 stages, though there are of course certain individual differences 

 due to different conditions of existence. The type, however, 

 always remains. 



Before fertilization can take place, ova and spermatozoa must 

 pass a maturing process which will be described on another 

 occasion. When fertilization is complete the ovum always 

 divides first into two, then into four, eight, &c., cells, forming the 

 morula or cell-heap. By further cell-multiplication is formed 

 the blastula, a minute hollow sphere the walls of which consist 

 of one layer of firmly united embryonic cells. Whether we have 

 so far observed a sponge, an echinoderm, a tunicate, or even 



