STRUCTURE OF THE EGG. 5 



Food-yolk plays so important a part in the development of 

 animals that it is well to consider its influence in some detail. 



The amount of food-yolk present is the main factor in deter- 

 mining the size of the egg ; and the differences in size between the 

 eggs of the cod and of the dogfish, or between those of Amphi- 

 oxus, the frog, and the hen, depend almost entirely on the fact 

 that the cod's egg contains but little food-yolk, and that of the 

 dogfish a great deal ; and that the egg of Amphioxus is almost 

 devoid of food-yolk, while the frog's egg contains a considerable 

 amount, and the hen's egg an enormous quantity. The size of 

 an egg depends on the amount of food-yolk present in the egg, 

 and not on the size of the animal that produces the egg. A cray- 

 fish lays larger eggs than a lobster, although the adult crayfish is 

 not more than a third the length of the lobster ; a cuckoo lays 

 much smaller eggs than other birds of its own size ; and a rabbit 

 is developed from an egg less than a sixteenth the diameter of a 

 frog's egg. 



The amount of food-yolk determines the actual size, and the 

 degree of development, which the embryo is able to attain at the 

 expense of the egg itself. If the quantity of nutritive material 

 within the egg is small, then it will be quickly absorbed, and 

 the young animal must hatch early, and consequently of small 

 size and imperfect development. If there is a greater amount 

 of food-yolk present, then a larger proportion of the develop- 

 mental history can be completed before the time of hatching ; 

 while in cases where the eggs are of great size, owing to great 

 abundance of food-yolk, practically the whole development can 

 be effected at the expense of the egg itself, and the young 

 animal hatches in the form of the parent. 



Amphioxus lays eggs which measure only about ^(jth inch 

 in diameter, and the young embryos consequently hatch of 

 very small size and in a very immature condition. The frog 

 lays larger eggs, about y^th inch in diameter, which contain 

 sufficient food-yolk to carry the embryo up to the tadpole stage 

 before hatching ; though the rest of the development, from the 

 tadpole to the frog, must be completed at the expense of food 

 obtained by the tadpole during its free living existence. A 

 hen's egg, on the other hand, is large enough, i.e. contains food 

 enough, to enable the embryo to proceed much further in its 

 development before hatching ; and the young chick leaves the 



