26 GENERAL EMBRYOLOGY 



familiar theory is, that the organism, in its individual devel- 

 opmental history, tends to repeat in outline the evolutionary 

 history of its species. This repetition is seldom particular, or 

 detailed, never complete, yet so many of the phenomena of 

 development can be satisfactorily interpreted from this his- 

 torical point of view, seeming to have this historical significance 

 rather than an immediately adaptive relation, that as a general 

 statement the law remains fundamentally true. 



This law is not so much an attempt to resume the facts of 

 embryology as to apply these facts in the interpretation of racial 

 history (Evolution). This application is in many instances 

 difficult because of the fact that there has been an evolution of 

 the egg, the embryo, and the larva, just as of the adult. The 

 fact that the organism is specific at all stages of its existence, 

 includes the parallel evolution of ova, and of all succeeding 



Species Zygote Stages in Development Adult 



D A D B D C D D 



E A E B E C E D E E 



F A F B F C F D F E F F 



G A G B G C G Do E a F G G 



H A H B u C H D H E H F H G H H 



FIG. 13. Diagram to illustrate the essentials of the Biogenetic Law. Modified 



from O. Hertwig. 



developmental stages, if there is to be any evolution of adult 

 structures, else diversity of adult organization would depend 

 upon external conditions of development, rather than upon egg 

 organization. But we know that the eggs of any species of 

 sea-urchin and star-fish will develop, respectively, into adult 

 sea-urchins and star-fish of those species, although in the same 

 dish, with identical environing stimuli. 



Many important points concerning the relation between 

 ontogeny and phylogeny may be represented schematically, 

 as in the accompanying diagram (Fig. 13). Here we com- 



