CHAPTER XXVII 

 THE DEVELOPMENT OF THE FROG'S EGG 



All living matter has the capacity of increasing itself under 

 proper contlitions to an almost unlimited extent, the food which 

 animals devour being the material out of which the new living 

 substance is made. This living substance exists in isolated 

 particles, or masses, which we call individual animals or plants. 

 The animal or plant has also at any stage a definite form which 

 is not exactly alike in any two individuals, but is roughly alike 

 within the " species." Now the number of individuals of any 

 species ' tends constantly to diminish through death. It is 

 actually maintained by reproduction, by a piece of the parent 

 individual being cut off to form a new individual. This piece 

 may be at first almost shapeless or approximately spherical. 

 But as it grows larger it assumes more and more the form and 

 complex structure of the adult. This process of growing into 

 the adult form is development. In most of the more familiar 

 cases development begins with an approximately spherical egg. 



In the case of the frog, the egg is between one and two mil- 

 limetres in diameter. The eggs, which are numerous, are laid 

 in a common jelly, and during development float near the sur- 

 face of the water in which they are laid. The first visible 

 changes are furrows running across the surface of the egg, as 

 a result of which the egg becomes divided into small areas 

 which correspond to a division of the whole egg into " cells." 

 This stage of development is called cleavage. '-^ Toward the end 



1 See also Chapter XVIII. ' Fig. 40S X-F. 



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