of the Annelids and Vertebrates. 471 



of the nervous system is developed extremely early, long 

 before the appearance of the gullet ; it finds no obstacle to its 

 growth forwards and above the rudimentary intestine, but 

 space enough to develop, extend, and establish itself. When, 

 then, later the mouth comes to be formed, it cannot break 

 through at the same point as in the Annelids ; for the cephalic 

 part of the nervous system here offers far too much resistance, 

 partly through its own nature and partly owing to the rapid 

 development of the embryonic skeleton around it. It is pos- 

 sible that, as Dohrn suggests, the sinus rhomhoideus indicates 

 the place where such a bi'eaking-through should have occurred, 

 and possible also that the new mouth, now appearing upon 

 the opposite side, is the result of a transformation of the first 

 gill-cleft. These are hypotheses which can scarcely ever be 

 really tested. It is sufficient that Dohrn and I agree that the 

 mouth of the Vertebrates occupies a different position from that 

 of the Annulates. Whether, as I believe, it is a fresh forma- 

 tion on the dorsum of the latter, because the primitive point 

 of perforation is rendered imjjassable owing to the great deve- 

 lopment of the brain, or whether it arises directly through the 

 transformation of organs already existing in this position, is 

 of no consequence for the questions immediately before us. 



The sole really morphological and effective argument, there- 

 fore, which Baer can adduce in support of his opinion, is the 

 position of the mouth, which, however, is not difficult to ex- 

 plain in the manner first suggested by Dohrn. Further, if 

 one reflects that in the type of the lladiates the position of 

 the mouth, as determined by the relation of the animal to 

 the surface supporting it, may be extremely variable, it will 

 scarcely be difficult to conceive it as situated in the one case 

 on the dorsum and in the other on the ventrum. 



If one does this, and then inverts the Annelid, a budding 

 Nais for example, so that its physiological dorsum lies down- 

 wards, there appears an almost absolute identity in the origin 

 and position of the individual organs of the Vertebrates and 

 Annelids. I will here enumerate these points once more, 

 although almost two years ago, and before any one else, I 

 brought some of them prominently forward. 



1. The central nervous system is developed unsegmentally 

 from the ectoderm. 



2. The spinal ganglia appearing from before backwards, 

 and developed out of the protoscgraents of the mesoderm, 

 unite with it. 



3. The ventral cord in the body of all Articulates has spinal 

 nerves with two roots, as in the Vertebrates. 



4. The dorsal oesophageal ganglion of tlie Articuh\tes does 



