58 DOCTRINE OF EVOLUTION 



would find somewhere among the leaves and sticks in 

 the water large masses of a clear jellylike consistency 

 enclosing hundreds of little black spheres about an 

 eighth of an inch in diameter. These are the egg 

 masses and eggs of a common frog. Watching them 

 day by day we see the small one-celled egg spheres 

 divide into more and more numerous portions which 

 are the daughter-cells, destined to form by their prod- 

 ucts the many varied tissues and organs of the develop- 

 ing larva and adult frog. After three or four days the 

 egg changes from its globular form into an oval or 

 elliptical mass, and from one end of this a small knob 

 projects to become a flattened waving tail a few days 

 later. On the sides of the larger anterior portion 

 shallow grooves make their appearance and soon break 

 through from the throat or pharynx to the exterior as 

 gill-slits. Shortly afterwards the little embryo wriggles 

 out of its encasing coat of jelly, develops a mouth, and 

 begins its independent existence as a small tadpole, with 

 eyes, nasal and auditory organs, and all other parts that 

 are necessary for a free life. Thus the one-celled egg 

 has transformed into something that it was not at first, 

 and in doing this it has proved the possibility and the 

 reality of organic reconstruction. 



The tadpole breathes by means of its gills, and it is 

 at first entirely devoid of the lungs which the adult 

 frog possesses and uses. When we speak of the larval 

 respiratory organs as gills we imply that they are like 

 the organs of a fish which have the same name ; they 

 are truly like those of fishes, for the blood-vessels 

 which go to them are essentially the same as in the lower 

 types and they are supported by simple skeletal rods 



