K. E. VON BAER. PHILOSOPHICAL FRAGMENTS. 235 



causes this separation. It is the animal part which grows so 

 much, that we observe the marking off of the embryo from the 

 germinal membrane. It is only after this has determined the 

 whole form of the animal, that the plastic part appears to attain 

 a certain degree of independence, which in the Articulata is often 

 limited to a mere separation, the separate organs arising subse- 

 quently, but which in the Vertebrata has sufficient power to 

 cause a symmetrical development of the animal part. Of the 

 action of the plastic part upon the animal, we can but detect a 

 trace here and there. It is otherwise in the Mollusca. The 

 plastic part becomes independent very soon, and has a decided 

 influence upon the external form. We see how the essential 

 character of the animal manifests itself very early, and develop- 

 ment affords a justification for the name of Plastic animals which 

 has been given to the Mollusca. Hence also we shall be better 

 able to judge how far the Mollusca may be justly compared with 

 the vegetative section of the body of the Vertebrata ; in its pre- 

 dominant character namely, not according to the sum of all its 

 separate parts. In the Mollusca, it is also a relatively animal 

 part which occupies the whole periphery, and is principally 

 developed in the foot of the Gasteropoda. Compared with 

 other animals, they are living bellies ; but since these bellies are 

 independently developed, without the influence of a more highly- 

 organized animal part, they have also a part which for them is 

 more animal, and it is that which originally formed that surface 

 of their germ which was turned from the yelk. 



In all four forms the surface of the germ which is turned 

 towards the yelk does not change its position relatively to the 

 latter, but retains it, and becomes the digestive surface of the 

 perfect animal. In all forms, moreover, the peripheral part of the 

 perfect animal is the external surface of the germ, that which is 

 turned away from the yelk. I therefore believed myself to be 

 justified in supposing that it is the relation to the yelk which in 

 the germ produces the primary differentiation into an animal 

 and a plastic layer. 



But it is not in all animals that the whole outer layer of the 

 germ remains exterior. In the Vertebrata, the one half of the 

 doubly symmetrical development encloses a part of the outermost 

 surface, and changes into the nervous tube, the spinal cord with 



