K. E. VON BAER. PHILOSOPHICAL FRAGMENTS. 211 



subsequently those which mark the smaller, divisions of the 

 Vertebrata. 



Thus the more special type is developed from the more general. 

 Every step in the development of the Chick testifies to this. In 

 the beginning, when the dorsal folds have closed, it is a verte- 

 brate animal, and nothing more. As it raises itself from the 

 yelk, as the gill-plates coalesce and the allantois grows forth, it 

 shows itself to be a vertebrate animal, which cannot live freely 

 in water. Subsequently the two caeca grow forth, a distinction 

 appears between the pairs of extremities, and the beak is deve- 

 loped ; the lungs pass upwards ; the rudiments of the air-sacs 

 are recognizable, and there can be no doubt as to its being a 

 Bird. While the ornithic characters become more and more 

 marked, in consequence of the further development of the wings 

 and air-sacs, of the coalescence of the tarsal cartilages, &c., the 

 web of the feet disappears, and we recognize a land Bird. The 

 beak, the feet, pass from the general into a special form ; the 

 crop is developed, the stomach has already divided into two cavi- 

 ties, and the scale of the nostrils appears. The Bird takes on the 

 character of a Gallinaceous bird, and finally of a domestic fowl. 



It is an immediate consequence, in fact it is merely a changed 

 form of expression of what has been said above, that the more 

 different two animal forms are, so much the further back must 

 their development be traced, to find them similar*. To demon- 

 strate that this is true, not merely for the Vertebrata, we will 

 select a few examples from the lower animals. The difference 

 between the Macrurous and Brachyurous Crustacea is not 

 very great. Now the river Crayfish has at the middle of its 

 embryonic life a tail tolerably short in relation to its broad tho- 

 racic segments, and it can be hardly distinguished from a Bra- 

 chyurous Crustacean, since, according to Cavolini's representa- 

 tion, these have in their embryonic condition tolerably long tails. 

 The further we trace them back, the more similar do we find the 

 jaws in Crustacea to the feet; in truth, they are at first the 

 anterior feet, and nothing else. We have therefore not only an 

 approximation to the fundamental type (agreement of allied or- 

 gans), but also a similarity with the Stomapoda, Amphipoda 



* This remark does not invalidate what has been said in the First Scholium 

 as to the indefiniteness of the same form in its earliest condition. 



14* 



