THE DEVELOPMENT OF THE FROG 283 



breviated, and profoundly modified. Organs which appeared 

 late in the history of the species may appear precociously in 

 the embryo; many of the steps of evolution are dropped 

 altogether out of ontogeny, and many new structures and 

 adaptations are intercalated, which can have no phylogenetic 

 significance. At the best, then, embryology is only an aid 

 to the interpretation of the history of animal organisation, but 

 taken in conjunction with the comparative anatomy of adult 

 forms, it is a very powerful aid indeed. And the student 

 should bear in mind that the life-history of every animal is, 

 in the strictest sense of the word, an evolution, a gradual 

 becoming of the evidently complex and heterogeneous from 

 the apparently simple and homogeneous fertilised ovum. Nor 

 should it be forgotten that, simple and homogeneous as the 

 ovum appears to us, it contains within itself the potentiality 

 of the future animal. Many ingenious theories have been 

 framed which, by attributing a great complexity of structure 

 to the germ - cell, undertake to explain the manner of its 

 development into an adult animal. But they are necessarily 

 extremely hypothetical and speculative, for observation shows 

 no complexity beyond that of an ordinary undifferentiated 

 cell, a corpuscle of protoplasm containing a nucleus. None 

 the less, we cannot escape the conviction that the constitu- 

 tion of the germ-cell cannot be so simple as it appears to 

 us, but must be of a kind adequate to produce the observed 

 phenomena of development. 



