296 THE RECAPITULATION THEORY 



stages of development can be determined by simple 

 inspection of the adult shell. 



Jt is important to remember that the Recapitula- 

 tion Theory, if valid, must apply not merely in a 

 general way to the development of the animal body, 

 b < ut~nmst hold good with regard to the formation of 

 each organ or system, and with regard to the later 

 e_qually with the earlier phases of development. Of 

 individual organs, the brain of birds has been 

 already cited. The formation of the vertebrate liver 

 as a diverticulum from the alimentary canal, which 

 is at first simple, but by the folding of its walls 

 becomes greatly complicated, is another good 

 example ; as is also the development of the vomer 

 in Amphibians as a series of toothed plates, equiva- 

 lent morphologically to the placoid scales of fishes, 

 which are at first separate, but later on fuse 

 together and lose the greater number of their 

 teeth. 



Concerning recapitulation in the later phases 

 of development and in the adult animal, the mode 

 of renewal of the nails or of the epidermis generally 

 is a good example, each cell commencing its exist- 

 ence in an indifferent form in the deeper layers 

 of the epidermis, and gradually acquiring the adult 

 peculiarities as it approaches the surface, through 

 removal of the cells lying above it. 



.The above examples, selected almost haphazard, 

 will suffice to illustrate the Theory of Recapitulation, 

 of the theory depends chiefly on its 

 unversaap^licability to all animals, whether high 

 or low in the zoological scale, and to all their parts 



