216 K.E.VON BAER. PHILOSOPHICAL FRAGMENTS. 



In detail it holds good as little as any other representation of 

 organic relations upon a surface. Thus the single features ad- 

 duced must pass for the whole characters, e. g. the formation of 

 wings and air-sacs for the whole character of Birds. The expo- 

 sition, again, can only be very imperfect, since for most animals 

 the investigation has hardly been commenced. 



This scheme is only meant to bring clearly before the mind, 

 how the first decisive distinction is whether the first rudiment 

 is a true egg or a germ-granule ; how, in the germs of ova, all 

 animals are at first alike (see above, e) ; how then the principal 

 type becomes defined (which is called, origin of the embryo) ; 

 whereby it remains undecided \vhether any radiate animal is 

 developed from a true egg. If now the type of the vertebrate 

 animal appears, the embryo is at first nothing but one of the 

 Vertebrata without any particular characteristics. Chorda dor- 

 salis, dorsal and abdominal tubes, gill-clefts, gill-vessels, and a 

 heart with a single cavity, are formed in all. Then commences 

 a differentiation. In a few, gill-laminas and no allantois are de- 

 veloped ; in others, on the other hand, the gill-clefts coalesce, 

 and an allantois buds forth. The former are aquatic animals, 

 though not all permanently so : the others lead an aerial exist- 

 ence. The latter all acquire lungs. Let us follow out the 

 former series first however. The embryos for a long time retain 

 a great similarity ; they push out long tails and scull about with 

 them in the water. On the other hand, their extremities are 

 developed very feebly and late, in relation to those of other em- 

 bryos. They either never acquire true lungs, and so become 

 fish, or else true lungs are formed. Among the latter the lungs 

 are either feebly developed, in which case the gills are permanent 

 and the animals become Sirenidae ; or the lungs are better 

 formed, and the gills either remain free until they cease to act 

 (Salamanders), or they become covered over, the tail disappears, 

 and with it all resemblance to a fish (tail-less Batrachia). In 

 the second series of the Vertebrata, which never has external 

 gills, the most essential distinction is perhaps this, that in 

 some a simple umbilicus is formed (Reptiles and Birds), in 

 others this umbilicus is prolonged into a cord, after, as it seems, 

 being altogether more rapidly formed (Schol. II. b.}. 



In what manner Birds become separated from the Amphibia 



