K. E. VON BAER. PHILOSOPHICAL FRAGMENTS. 233 



There remains yet another agreement between all forms of 

 development. In all animals, namely, which at an early period 

 possess a germ and a yelk, the investing germ becomes divided 

 into many layers ; that turned towards the yelk is the plastic 

 receptive layer; that turned from it, the more animal, even 

 though its outermost border should become merely a limiting 

 organ, and clothe itself more or less with an excreted non-vital 

 layer. That now the vascular system, if it is otherwise sepa- 

 rated from the digestive cavity, is formed external to it nearer 

 to the animal part ; that in the animal part, muscles, nerves, &c., 

 become separated, appears also to belong to the Idea of the ani- 

 mal in general ; and the further this histological differentiation 

 goes, the more developed do we call an animal. 



Wholly different from this, however, is the relative position 

 of the parts. This is determined by the external form of the 

 development. 



We have distinguished four principal forms, or, as we have 

 called them, Schemata of Development : 



Radiate Development (Evolutio radiata), which, proceeding 

 from a centre, repeats similar parts peripherally. 



Coiled Development (Evolutio contorta), in which similar parts 

 are twisted round a cone or other space. 



Symmetrical Development (Evolutio gemma], in which similar 

 parts are distributed from an axis on both sides to a sutural line 

 opposite to the axis. 



Double Symmetrical Development (Evolutio bigemina], in 

 which from an axis similar parts are disposed on both sides 

 above and below, and are united at two sutural lines ; so that 

 the inner layer of the germ is closed below and the upper layer 

 above. 



We know that in the higher Vertebrata the Germ soon divides 

 into two parts, an inner, which may especially be termed the 

 Embryo ; and an outer, which may be called the Germinal mem- 

 brane. I have already remarked that the former is nothing but 

 a part of the germ, which is metamorphosed according to the 

 scheme of development peculiar to every animal, whilst the peri- 

 pheral part remains behind in its development. 



In Mammalia, Birds, and Reptiles, the middle part is but 

 small in proportion to the outer portion, and it gradually sur- 



