K. E. VON BAER. — PHILOSOPHICAL FRAGMENTS. 193 



forms), because their testes are contained in the abdomen, be- 

 cause some of -them have no true teeth, because the anterior and 

 posterior sphenoids remain separated, &c. &c. But the other 

 cranial bones of the Cetacea unite very early and very closely, 

 and therefore are adult-like. Their jaws are very long, although 

 all Mammalia, the Cetacea included, have their jaws the shorter 

 the younger they are. The separation of the cranial bones, how- 

 ever, is no especial peculiarity of the embryonic condition, which 

 is absent in the adult condition of the lower classes of animals ; 

 for in Fishes it is brought forward again as an embryonal con- 

 dition that the cranial bones are divided into many portions, and 

 are merely applied to one another, although at the base of the 

 skull the unity of the sphenoid — the very opposite of what occurs 

 in the Cetacea— offers, on the other hand, a resemblance with 

 the adult condition of the highest Mammalia. Agreement with 

 a Fish or with a Cetacean is therefore no absolute condition for 

 the organization of the embryo. 



4. If the law we are engaged in investigating were correct, no 

 conditions which are permanent only in the higher animals 

 could be a transitory stage in the development of particular 

 lower forms. But a great number of such conditions are de- 

 monstrable. We cannot indeed discover them in the course of 

 human development, since we know no higher organization. 

 But the Mammalia afford examples enough. In all the jaws are 

 at first as short, as they are permanently in Man ; the parietal 

 ridge is developed very late in animals which are provided with 

 it, while, on the other hand, it is wanting in the highest forms. 

 Instances of this kind multiply the further we descend. We 

 have already introduced Birds speaking, in order to insist upon 

 a multitude of previously-known relations in which the embryo 

 of the Bird agrees with the adult Mammal. We can bring for- 

 ward still more. The brain of Birds in the earliest third of em- 

 bryonic life is much more similar to the brain of Mammals than 

 in the adult condition. The corpora quadrigemina have not de- 

 scended, the olfactory bulb is hollow and thick, and there is 

 even a kind of fornix present. The heel of the Bird developes 

 itself from many cartilages into a single bone. The eyes in the 

 Chick are at first placed nearer together than subsequently, and 

 give it a humanized face. Young Lizards have a very large 



SCIEN. MEM,— Nat. Hist. Vqj-. I. Part III. 13 



