K. E. VON BAER. PHILOSOPHICAL FRAGMENTS. 21? 



has already been shown. Probably a difference also arises very 

 soon in the vascular system, whose metamorphoses in the Am- 

 phibia, however, are not yet known. While the gill-clefts in 

 the Lizards are still open, the heart has just the same appearance 

 as in Birds at the same period. Now just as in the Bird the 

 special characters of the family and of the genus arise, so is it in 

 the Mammalia. The Dog and the Pig are at first very much 

 alike, and have short human faces. Still longer does the resem- 

 blance persist between the Pig and the Ruminant, whose lateral 

 toes are at first almost as long as the two median ones. For the 

 rest we are by no means sufficiently acquainted with the em- 

 bryos of the Mammalia to state how and at what periods they 

 become distinguishable from one another. We are best ac- 

 quainted with the differences in the form and structure of the 

 ova. Since these are very manifold in their form and in their 

 relation to the parent, I have ventured, in order not to leave the 

 Mammalia out of the Scheme, to divide them according to their 

 ova. The embryos, in fact, may be distinguished into those 

 which are born early and those which come into the world in a 

 fully developed condition. Among the former the ova of the 

 Monotremata are probably born undisturbed. In the Marsu- 

 pialia the embryo has burst its membranes. The ova, which 

 are retained longer, may be reduced to three principal divisions. 

 In the first I place ova, in which the yelk-sac continues to grow 

 for a long time. They yield Mammals with narrow hook-like 

 nails (claws). In some the allantois is early arrested in its 

 growth, and the placenta is limited to one spot, or two-lobed* 

 (Rodentia). In others the allantois is developed to a moderate 

 extent (Insectivora) : in all others it grows over the whole am- 

 nion transversely, and the placenta is annular (Carnivora). A 

 second division of long-retained ova is formed by those in which 

 the yelk-sac and the allantois are small; the placenta is one- 

 sided, and is, as it would seem, in the opposite position to that of 

 the Rodentia; the amnion and the umbilical cord are here 



* I must for the present follow Cuvier in stating that in the Rodentia the 

 allantois remains very small, since in my earlier investigations I did not pay 

 sufficient attention to this point, and for the last three months I have endeavoured 

 in vain to obtain pregnant Rabits. The allantois is moderately large in the 

 Hedgehog (one of the Insectivora), as I have recently observed. 



