218 K. E. VON BAER. PHILOSOPHICAL FRAGMENTS; 



largest. These ova produce animals with flat nails and three- 

 lobed cerebral hemispheres*. A third division has a yelk-sac 

 which soon disappears, but an allantois which grows outimmensely 

 at its two extremities. These ova produce ungulated and finned 

 animals ; if the placenta is distributed over the whole ovum, but is 

 collected in particular masses, we have animals with cleft hoofs ; 

 if it is distributed homogeneously, we have other Ungulata and 

 Cetaceaf. Hence the principal differences of the Mammalia are 

 marked very early in the ovum, for according as the allantois is 

 much developed, or otherwise, does the ovum become long or 

 short J. In the former case, the embryo not only acquires a 

 broader horny covering upon its fingers, but also a more com- 

 plex stomach, and, in connexion therewith, long jaws, a flat 

 articulation of the jaw, usually complex teeth, incapability of 

 seizing and climbing, &c. &c. It is the plastic series among the 

 Vertebrata. 



I must advert to an objection against the whole view here set 

 forth, which may be based upon the circumstance that in some 

 cases the embryos of nearly allied animals exhibit considerable 

 differences at an early period. The embryos of the Ophidia, for 

 instance, are very early rolled up, and so may be readily enough 

 distinguished from Lizards. This plainly arises from the ex- 

 cessive length to which in this case the vertebrate type is drawn 

 out. 



Dissection, however, exhibits a great harmony in the internal 

 structure ; and since the posterior extremity of the Lizards also 

 forms a spiral, the difference probably lies merely in this, that 

 the vertebrate type in the Ophidia is more elongated, and it 

 seems, in fact, to be greater than it is, because it presents itself 

 so nakedly. Thus also the larvaj of many families of Insects are 

 in their external appearance very different in different families. 

 Much probably depends in this case upon their shorter or longer 

 sojourn in the egg. However, this objection, the only one which 



* It would be very interesting to know the ova of the Lemurs, so as to ascer- 

 tain whether they are very similar to those of the Monkeys or not. 



t According to a letter of Rudolphi's, the chorion of the Dolphin is similar 

 to that of the Horse. According to Bartholin it is a. placenta exilis. 



X Perhaps the difference may be recognized still earlier in the chorion. See 

 Ueher die Gefiissverhindung zwischen Mutter nnd Frucht. Leipzig, 1828, fol. 



