TYPES OF DEVELOPMENT. 49 



and in a certain sense, by a particular gradual evolution 

 of the organs, a bird also, but made the embryo like- 

 wise repieat and surpass the lower types. To this false 

 tendency, acting on vague analogies, a stop was put by 

 the great naturalist just named. He showed that a 

 number of coincidences might, indeed, be demonstrated 

 between the embryo of the higher and the permanent 

 form of the lower animals, but that this resemblance 

 rested essentially on the fact that in the embryo of the 

 higher animal the differentiation of the general funda- 

 mental mass had not yet set in, and that in the progress 

 of development it passes through stages which are per- 

 manent in the series of inferior animals. 



On the other hand, he positively repudiated the asser- 

 tion that the embryos of the higher types actually pass 

 through forms permanent in the lower ones. He says 

 that the type of each animal seems from the first to 

 fix itself in the embryo, and to regulate its whole 

 development. As regards the vertebrate animals in 

 particular, the further we go back in the history of their 

 development, the more do we find the embryos alike, 

 both on the whole and in the individual parts. " Only 

 gradually do the characters appear which mark the 

 greater, and later those which mark the smaller divi- 

 sions of the Vertebrata. Thus from the general type 

 the special one is evolved." 



Von Baer thus held that the analogy consisted only in 

 the embryonic states of the various animal forms ; but he 

 was obliged to go beyond the circle of the types, and he 

 thought it probable that among all embryos of verte- 

 brate, as well as invertebrate animals, developed from a 

 true ovum, there is a conformity in the condition of the 



