196 VERTEBRATES. 



The cranium is of cartilage when first formed, and is 

 not completely ossified in old specimens, as may be seen 

 from the large spaces left in its lateral walls in prepared 

 skeletons. 



Make an enlarged, and if necessary diagrammatic, draw- 

 ing of the frog's skull as seen from below, naming all the 

 bones visible in the drawing. 



Development. Every student should study the life 

 history of the frog. In the spring the eggs may be easily 

 obtained from shallow pools in which frogs are heard 

 croaking. Being as large as peas, and each with a dark- 

 colored yolk surrounded by a layer of whitish albumen, 

 and all suspended in large lumps of transparent, jelly-like 

 substance, they are conspicuous and peculiar appearing 

 objects. They may be looked for at the edges of the 

 pools, amid the trash which collects there, and often partly 

 hidden by it. These egg masses are popularly known as 

 frog spawn. 



The eggs, if carried home or to the laboratory, and 

 kept in clean, cool water, will go on developing. If 

 freshly laid eggs have been obtained, segmentation may 

 be seen taking place in the yolk. Later, as the embryo 

 develops, the yolk will appear oval in outline ; and then, 

 elongating more and more, an unmistakable head and tail 

 will soon appear. At length the embryo becomes active, 

 and breaks through the jelly-like mass, and is hatched. 

 It attaches itself for a short time, by means of a pair of 

 minute suckers near the mouth, to the jelly-like mass or 

 to plants, but soon becomes a free-swimming tadpole. It 

 then breathes by means of three pairs of external gills, 

 which appear as minute tufts at the sides of the head. 

 The water passes in at the mouth, and out through three 

 pairs of gill slits located just in front of the gills. 



If one of these small tadpoles be placed in a watch glass 



