DEVELOPMENT OF EGGS IN MANY-CELLED ANIMALS 347 



with the developing embryo for its use. If these stores of food 

 material are abundant, the changes which go on within the egg, as 

 we may term the egg-cell together with accessory structures, may 

 take a considerable time, and the young animal may hatch out in 

 a form not very unlike that of the adult. A familiar example is 

 afforded by the life-history of a Fowl. The egg is of large size, 

 and made up of the developing egg-cell with the nutritive matter 

 stored up in it (the " yolk "), of the extra food-supply known as the 

 albumen (the " white "), and of two protective coverings, i.e. the 

 double shell-membrane and the porous calcareous shell (fig. 866). 

 As everyone knows, the chick which emerges from the egg after 



GERMINAL DISC 



YELLOW YOLJIi 



WHITE YOLK- 



SHELL 

 MEMBRAME 



WHITE OR. 

 ALBUMEN 



Fig. 866. — Diagrammatic Longitudinal Section through a Hen's Egg 



three weeks' incubation is at once able to run about, and soon 

 begins to pick up food. 



On the other hand, hatching-out is a comparatively rapid 

 process in the case of eggs which contain a small amount of 

 nutritive matter, and the young animal is consequendy in a very 

 immature condition when it is thus prematurely forced to get its 

 own living. It may often be described as a larva, which is very 

 unlike the adult stage to which it ultimately attains. We have 

 seen, for example, that a typical sponge is hatched out as a little 

 ciliated oval Planula, and it may be added that the young Lancelet 

 does not at first resemble the parent form. These, however, are 

 somewhat extreme cases. Larval forms may result from the 

 hatching of eggs containing a fairly large amount of yolk, as 

 in the Frog. And it does not follow that the presence of large 

 stores of nutritive material in the egg is correlated with such 

 perfectly-formed young as those characteristic of Fowls. The 

 nestlings of a Sparrow, for instance, are particularly helpless, and 



