Top view after 

 First division second division 





-■^ 



Two -layer cup 



Caving in 



Section of 



hollow sphere 



EARLY STAGES IN THE DEVELOPMENT OF A FROG 



The yolk material is heavier than the protoplasm and remains at the bottom of the 

 mass. When a cell division takes place in a horizontal plane, the upper cells are 

 smaller and more active, and the lower ones, with more inert food material, larger and 

 less active. At one time the frog is a hollow sphere; at another, a two-layer cup 



hatches out, the young animal looks more like a worm than like the parent. 

 It has no wings, as has the adult. Its biting jaws work sideways. It differs 

 from the adult so much that we should never suspect its connection with 

 butterflies if we did not observe its origin or its later development. In the 

 life history of frogs and salamanders there are also distinct stages, in some 

 ways as well marked as those of the insects (see illustration, p. 355). The 

 development of an individual through a series of well-marked stages is 

 called a metamorphosis, which means "trans-formation". 



Diflerentiation' There is another way of looking at the process of 

 development. As the mass grows and as it undergoes changes in form, the 

 cells become more and more unlike the original cell from which their 

 growdi started. They also become more and more unlike one another. The 

 skin and muscle cells become distinguishable from the bone and nerve cells. 

 The cells of the stomach glands become different from those of the saliva 

 glands. Cells come to differ from one another in size, in shape, in coloring, 

 in texture, and in their chemical peculiarities. There is a progressive dif- 

 jerentiation. Growth, differentiation, metamorphosis, are various aspects of 

 the same general fact of development, which is characteristic of all livina 

 things. 



^See Nos. 5 and 6, p. 365. 

 351 



