K. E. VON BAER. — PHILOSOPHICAL FRAGMENTS. 213 



Gasteropods has an unequal coloration upon its surface. Be- 

 sides I have seen clearly the germinal layer and the germinal 

 vesicle, the predecessors of the germ. It therefore seems to me 

 very probable that all true ova have a distinct germ. 



The further back we trace development, so much the more 

 agreement do we find among the most widely different animals, 

 and thus we are led to the question, — Are not all animals essen- 

 tially similar at the commencement of their development — have 

 they not all a common primary form ? We have just remarked, 

 that a distinct germinal disc probably exists in all true ova ; so 

 far as we are acquainted with the development of germ-granules 

 (Keim-korner), it seems to be wanting in them. They appear 

 to be originally solid; however it may be, that on their first 

 separation from their parent, they have an internal cavity like 

 the central cavity of the yelk, which only escapes microscopic 

 observation on account of the thickness of the often somewhat 

 opake wall. Supposing, however, they are at first solid, and 

 eventually become hollow, as seemed to me to be the case with 

 the germ-granules of the Cercariae and Bucephali*, yet we 

 perceive that the first act of their vital activity is to acquire a 

 cavity, whereby they become thick-walled, hollow vesicles. The 

 germ in the egg is also, according to Schol. II. c, to be regarded 

 as a vesicle, which in the Bird's egg only gradually surrounds 

 the yelk, but from the very first is completed as an investment 

 by the vitellary membrane ; in the Frog's egg it has the vesicular 

 form before the type of the Vertebrata appears, and in the Mam- 

 malian from the very first it seems to surround the small mass 

 of the yelkf. Since, however, the germ is the rudimentary 

 animal itself, it may be said, not without reason, that the simple 

 vesicle is the common fundamental form from which all animals 

 are developed, not only ideally, but actually and historically. 

 The germ-granule passes into this primitive form of the inde- 

 pendent animal immediately by its own power ; the egg, how- 

 ever, only after its feminine nature has been destroyed by fecun- 

 dation (compare the Coroll. to Schol. I.). After this influence, 

 the differentiation of germ and yelk, or of body and nutritive 

 substance, arises. The excavation of the germ-granule is nothing 



* Nova Acta Acad. Nat. Cur. vol. xiii. T. 2. p. 658. 



t Heusinger's Zeitschrift fiir organische Physik, Bd. ii. p. 173. 



