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INTRODUCTION TO ZOOLOGY. 



divides into two cells nearly equal to each other in 

 size. These two cells divide again, thus forming four 

 cells nearly equal in size. This process continues, 

 each cell dividing into two, until the whole mass of 

 the egg consists of many cells. 



At this stage, and for some time afterward, there 

 is nothing in the appearance of the egg itself that 

 would indicate whether the egg is to develop into a 

 vertebrate or into an invertebrate. Its vertebrate 

 characteristics do not begin to appear until after it has 

 manifested the characteristics of an invertebrate 

 animal. In fact, we may say that at one stage of its 

 existence, the tadpole, or frog, manifests all the char- 

 acteristics of an invertebrate. For our present pur- 

 pose, we may say that it passes through the same 

 stages that a worm does, for we may take a worm as 

 being a typical invertebrate. 



Later, the vertebrate characteristics begin to ap- 

 pear, but it is not yet a frog. It begins to show red 

 blood, a two-chambered heart, and gills, and blood 

 goes from the heart to the gills just as it does in a 

 fish. In fact, it is a fish. Afterward the four legs 

 appear, while the tadpole still keeps its tail. If we 

 compare the tadpole in this stage with the salamander, 

 we see that the two are almost identical, both being 

 tailed Batrachians. But the tadpole goes on to an- 

 other stage. Its tail disappears and it becomes a 

 frog. 



We may arrange the steps as follows : — 



Table of Stages in the Development of a Frog. 



