REPRODUCTION 95 



does not feed, but it changes imperceptibly under the 

 cocoon into the adult stage, be it a beetle, butterfly, 

 moth, or other winged insect. 



We can pass over the embryonic development in 

 the next classes of animals, as they show no special 

 features to be recorded here. It is only when we 

 come to the frog that we find once more an interest- 

 ing case of metamorphosis. The frog deposits the 

 fertilized eggs, commonly called spaw r n, in large masses 

 in ponds and ditches. The spawn, together with the 

 yolk on which the young embryo feeds, is enveloped 

 in a gelatinous outer covering, w r hich serves as a 

 protection against danger. In about a week the egg 

 hatches out as a fish-like creature with head and 

 body all in one, and a long tail which is adapted for 

 swimming (Fig. 39). Little tufts of gills, which serve 

 for breathing, branch out at each side of the head. 

 It has now become a larva, which actively seeks its 

 food in the water, devouring almost any kind of 

 animal or vegetable matter. Soon internal gills are 

 formed, and as they develop the external gills decline. 

 The tadpole stage now follows. By much feeding 

 the body has become large and round, and the tail 

 has greatly increased in size. At the base of the tail 

 two legs have begun to bud out, while the two fore- 

 limbs, which are hidden within the gill chamber, have 

 also reached the same stage of growth. Within a 

 fortnight or so the tadpole undergoes great changes. 

 It ceases to swim about, and attaches itself to some 

 piece of weed in the water by a kind of sticky sucker 

 on the under-surface of the head. The outer skin 

 is then cast off and with it the external gills. The 

 fore-limbs appear, and the tail, which has served as 



