240 
R. Ruggles Gates. 
CHAPTER XI. 
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 (l.c., 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 
