K. E. VON BAER. — PHILOSOPHICAL FRAGMENTS. 223 



mode in which it is developed, its true import can only be re- 

 cognised by knowing the manner in which it is formed. At 

 present we judge for the most part in accordance with an inde- 

 finite feeling, instead of considering every organ only as an iso- 

 lated development of its fundamental organ, and determining 

 from this point of view the agreements and differences among 

 the different types. Every type has, in fact, not only its funda- 

 mental organs, but in each these are again divided into special 

 organs, which cannot be exactly what they are in any other 

 type. We therefore need some complete nomenclature, which 

 shall not merely apply the names of organs found in the verte- 

 brate type to the organs of other types, but shall give to them 

 special names when they have a different origin. This require- 

 ment will hardly indeed be satisfied in a century, but it is well 

 to call attention to it. In fact, the immediate consideration of 

 the perfect animal has often led to the recognition of the essen- 

 tial difference; perhaps, however, the determining conditions 

 have been less easily comprehensible. 



First of all, I would refer to the question how the series of 

 ganglia upon the abdominal side of the Articulata is to be 

 named. They certainly do not constitute a spinal cord, since 

 this is composed of a nervous tube, which can be produced only 

 upon that scheme of development, which is followed in the Ver- 

 tebrata. As little are they comparable to the sympathetic nerve 

 of the Vertebrata, for they supply voluntary muscles, and in the 

 Articulata the plastic nervous system lies upon the dorsal sur- 

 face*. They are rather the terminations of the symmetrical 

 nerves of animal life, and, on that very account, as Weber and 

 Treviranus have already remarked, they are comparable to the 

 nerves and ganglia called spinal, in the Vertebrata, on account 

 of their insertion into the spinal cord. In the Articulata, how- 

 ever, these nerves have only one series of central and peripheral 



* Some time ago, indeed, I was inclined to consider the so-called recurrent 

 nerves of Articulata to be their plastic nervous system, because I had traced 

 them a long way in the Crab ; however, I first became fully instructed on this 

 subject by a letter from Prof. J. Miiller. Miiller, by his exactness in investi- 

 gation and delicacy in dissection, has succeeded in following out these nerves 

 through the whole extent of the plastic organs ; and he has had the goodness 

 to communicate an excellent drawing of them to me. 



