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



rounds the yelk, forming the abdominal suture. In the Frog, 

 indeed, the external part of the germ is very thick, yet, as I 

 believe, a separation into embryo and germinal membrane is un- 

 deniable ; for the middle portion at the period in which the back 

 closes is still very much thicker, and the limitation is tolerably 

 well marked between the abdominal plates and that exterior por- 

 tion w hich I regard as the germinal membrane. The former 

 grow together towards the suture of the abdomen. In very 

 young Pikes, also, in which the germinal membrane is much 

 thinner and more transparent, this appeared to me to be the 

 case. 1 saw beside the trunk of the vertebral column a pair of 

 very narrow dark striae, the commencing abdominal plates. It 

 may hence, I believe, be affirmed universally of the Vertebrata, 

 that the embryo will surround the yelk with its abdominal plates, 

 although this has previously been invested by the germinal 

 membrane. It is the same in the Articulata. Their lateral 

 plates are clearly distinguished by their thickness from the 

 proper germinal membrane. They also surround the yelk. In 

 the Mollusks, however, the whole germ appears to change 

 equally. It cannot therefore be said of them that the embryo 

 grows round the yelk, but more justly, that from the moment of 

 fecundation it remains as its investment ; for a differentiation of 

 the germ into embryo and germinal membrane is not perceptible ; 

 the whole germ, rather, becomes the embryo. 



The same would very probably also take place in the radiate 

 type, if any animal form from this series were developed from a 

 true ovum, of which we have no knowledge*. If they should 

 all be developed from mere germ-granules, the relation is still 

 more apparent; for every germ-granule, so far as we know", is 

 developed as a whole, and is nothing but a germ without a 

 yelk. 



We must not overlook here an interesting relation. In those 

 ova in which the germ is clearly separated into an embryo and 

 a germinal membrane, it is the animal part of the embryo which 



* At present hardly anything could be of more interest for the history of 

 development than the observation of the development of the Asteridae, and 

 after these of the Cephalopoda. According to Cavolini, the latter have a yelk- 

 sac depending from the mouth {Abhandl. iiber die Erzeugung d. Fische und 

 Krehse uebers. von Zimmermann, 1792, p. 54), which is difficult to understand. 



