210 K. E. VON BAER. PHILOSOPHICAL FRAGMENTS. 



From this the dorsal plates elevate themselves, and the abdo- 

 minal plates soon arise, and the spinal cord becomes separated. 

 All these formative processes take place very early ; and it is 

 obvious that henceforward there can be no question of any 

 agreement with an invertebrate animal ; that, on the other hand, 

 those conditions which essentially constitute the vertebrate 

 animal are the first to arise ; the commencement of their deve- 

 lopment is however very similar in all classes of the Vertebrata. 

 We may therefore say, not merely of Birds, but more generally, — 

 The embryo of the vertebrate animal is from the very first a 

 vertebrate animal, and at no time agrees with an invertebrate 

 animal. A permanent animal form, however, which exhibits 

 the vertebrate type, and yet possesses so slight a histological 

 and morphological differentiation as the embryos of the Verte- 

 brata, is unknown. Therefore, the embryos of the Vertebrata 

 pass in the course of their development through no {known) per- 

 manent forms of animals whatsoever. 



Can, however, no law be discovered to regulate the develop- 

 ment of the individual as the possessor of a special organic form ? 

 I believe there can, and I shall endeavour to educe it in the 

 course of the following remarks. The embryos of Mammalia, 

 of Birds, Lizards and Snakes, probably also of Chelonia, are in 

 their earliest states exceedingly like one another, both as a whole 

 and in the mode of development of their parts ; so much so, in 

 fact, that we can oflen distinguish the embryos only by their 

 size. In my possession are two little embryos in spirit, whose 

 names I have omitted to attach, and at present I am quite unable 

 to say to what class they belong. They may be Lizards, or 

 small Birds, or very young Mammalia, so complete is the simi- 

 larity in the mode of formation of the head and trunk in these 

 animals. The extremities, however, are still absent in these 

 embryos. But even if they existed in the earliest stage of their 

 development, we should learn nothing ; for the feet of Lizards 

 and Mammals, the wings and feet of Birds, no less than the 

 hands and feet of Man, all arise from the same fundamental 

 form. The further, therefore, we recede in tracing the formation 

 of the Vertebrata, the more similar we find the embryos in their 

 totality and in their separate parts. At first those characters 

 gradually present themselves which indicate the greater, and 



