Recapitulatory and Karyogenetic Characters. 99 



CHAPTER XL 



GENERAL COMPARISON OF RECAPITULATORY AND 



KARYOGENETIC CHARACTERS. 



The Biogenetic Law. 



It is only necessary to refer to two recent discussions 

 of the biogenetic law in addition to those already cited. In his 

 excellent book on form and function, Russell (1916) has written a 

 history of animal morphology, including numerous references to the 

 biogenetic law. Those who have opposed the law appear to have 

 based their beliefs largely on (1) the dissimilarities found in related 

 embryos and eggs, (2) the fact that specific characters often make 

 their appearance very early in the ontogeny. Now both these 

 situations are to be anticipated if mutations have taken place in 

 organisms which already display recapitulatory characters. One 

 of the most striking cases of the appearance of specific characters 

 very early in the ontogeny, is cited by Russell (I.e., p. 352), who 

 quotes Louis Agassiz. The latter wrote in 1859 that the snapping 

 turtle " exhibits its small cross-like sternum, its long tail, its ferocious 

 habits, even before it leaves the egg, before it breathes through 

 lungs." It snaps at everything brought near, even when still 

 surrounded by its amnion and allantois. This is to be expected if 

 the specific characters in question have originated through 

 mutations, for it is now well-proven, in plants at least that 

 mutational characters begin to express themselves very early in the 

 ontogeny. And this is a natural result of the circumstance that 

 they are present in every nucleus. On the other hand, there is a 

 certain amount of embryological evidence in animals that characters 

 borne in the nuclei (in contrast to morphogenetic substances in the 

 egg cytoplasm) frequently became actuated only after the earlier 

 cleavage stages are passed. That such characters make their first 

 appearance earlier than some of the recapitulatory characters 

 which they traverse, is also to be expected ; but it only limits and 

 does not nullify the biogenetic law, since that law applies only to 

 recapitulatory characters but not to ordinary mutational characters. 



Sedgwick's (1894) criticisms of von Baer's law are based on the 

 same objections as those considered above. He compared the 

 embryos of the fowl and dogfish, admitting that they agree in many 

 important points, as the presence in the chick of pharyngeal clefts, 

 a tubular piscine heart, a similar arrangement of the cardiac 



