THE DEVELOPMENT OE THE FROG 81 



CHAPTER V 

 THE DEVELOPMENT OF THE FROG 



THE frog, like all higher animals which are developed 

 through sexual reproduction, begins its existence as a single 

 cell, the ovum or egg. The eggs or ova arise in the ovary, 

 and, when full grown, break out into the body cavity, where 

 they float around freely until they are carried by the action 

 of cilia into the funnel-shaped mouths of the oviducts. They 

 are then carried down the oviducts by the action of cilia 

 lining the walls and, during their passage, they become sur- 

 rounded by a thick coating of gelatinous substance. From 

 the oviduct they pass into the large, thin-walled uteri, where 

 they remain until they are discharged into the water, where 

 they are fertilized and undergo their development. 



There is perhaps no phenomenon in nature more marvel- 

 ous than the formation of a complex organism from a single 

 and apparently simple cell. At the one end of the process 

 we have a small mass of seemingly lifeless material with only 

 the simplest visible features of structure, and at the other 

 a creature with a complex organization composed of parts 

 beautifully coadapted and working in harmony, with numer- 

 ous instincts by which it adjusts itself to the various objects 

 in its environment, and most remarkable of all, with volition 

 and intelligence which, in the highest forms, may attain a 

 high degree of development. The production of even the 

 simplest animal seems almost a miracle, and our wonder is 

 only increased the more we attempt to understand how and 

 why the process takes place. 



