90 REPRODUCTION 



The first difference in these young cells showed at the 

 eight-celled stage when we had four small and four large 

 cells. At this time we know that the small cells are to 

 make the outer part of the skin, the nervous system, and 

 the sensitive parts of the animal generally. The larger 

 cells will give rise to the digestive tract and its glands. 

 These cells then are different, although they are all 

 descended from the same cell at the beginning. This is 

 called differentiation. The cells, from this time on, 

 differentiate more and more. Some of them become nerve 

 cells, some become bone, others become gland cells, 

 muscle cells, connective tissue, blood cells, pigment cells, 

 and so forth. 



In three or four days changes enough have occurred to 

 enable us to see at a glance which end is to be head and 

 which tail; which side is to be back and which belly; 

 which right and which left. In a few days more, the 

 young tadpole breaks out of the envelop of gelatin and we 

 say it is hatched. From this point on it leads a different 

 life, because its environment is greatly changed. 



5. The Life of the Tadpole. As the young frog, which 

 is called a tadpole, develops, it has organs suiting it to life 

 in the water. It has a soft skin that would dry up in a 

 few minutes in the air, it has gills by which it takes its 

 oxygen from the water, it has a fin-like tail by which it 

 swims freely, it has no legs that would enable it to walk. 

 In a word, it is a water-living, fish-like creature with no 

 adaptations whatever for an air and land life. During a 

 period varying from three or four months to two years, 

 depending on the species and the conditions of life, this 

 tadpole may continue. During this time it eats vegetable 

 matter. The tadpoles of the large species like the bull- 

 frog may become three or four inches in length. 



6. The Great Change. If the tadpole stopped here, his 

 history, perhaps, would be as interesting as most others. 

 He might be considered a strange kind of fish. But he 



