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 repeat 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 num- 

 ber of coincidences might, indeed, be demonstrated be- 

 tween 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 fundamental mass 

 had not yet set in, and that in the progress of development 

 it passes through stages which are permanent 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 par- 

 ticular, 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 divisions 

 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 



