Germinal Layers in Vertebrates, 375 



canal. This canal owes its origin to the genetic connexion 

 between the rudiment of the nervous system and that of the 

 notochord. 



(6) The fact that the rudiment of the notochord and of the 

 mesoderm arises as a continuous mass of cells from a rudi- 

 ment which also gives origin to the nervous system may be 

 turned to account for the purposes of phylogeny. It follows 

 from this that, in attempting to derive the bilaterally symme- 

 trica] Chordata from Coelenterate-like forms with radial 

 symmetry, we must remember that the notochord and the 

 mesoderm situated on either side of it in no way arise from 

 the dorsal portion of the intestine, but from the ingrowth of 

 the proliferating ectoderm cells. Since this ectoblastogenous 

 mesoderm is chiefly utilized for the formation of the muscu- 

 lature, we may apply the term ch or do-muscular to the entire 

 ectoblastogenous rudiment. Since this rudiment has a 

 common origin with that of the nervous system, the Chordata 

 exhibit a direct connexion with tlie Annelids, in which, 

 according to the investigations of Kleinenberg, the muscu- 

 lature also arises from the common neuro-muscular rudiment. 

 In my opinion the resemblance is still further increased by 

 the presence of the cephalic shield (or ventral shield, as the 

 case may be) in Annelids, which, in its position and its 

 relation to the nervous system and the muscles and in its 

 structure, is very similar to the notochord. It might then be 

 assumed that this continuous neurochordo-muscular rudiment 

 had already made its appearance in the animals which are to 

 be regarded as the common ancestors of the Chordata and 

 the Annelids. The notochord may have arisen in these forms 

 from an ectodermal rudiment as an axial structure which 

 served for the attachment of the musculature on the one side 

 and of the nervous system on the other. The further deve- 

 lopment and differentiation of different portions of this con- 

 tinuous ectodermal rudiment might have taken different roads. 

 In the Annelids the notochordal rudiment experienced no 

 further development. Perhaps, nevertheless, in the case of 

 certain Annelids a homologue of the notochord may be proved 

 to exist in the epithelial vesicular tissue *, which, in addition 

 to a certain similarity of structure, also exhibits some resem- 

 blance to the notochord in the fact that it serves for the 

 attachment both of the nervous system and of the lateral 

 musculature. 



The Ascidians have followed another path. In these 



* I shall later on make a special communication on the subject of this 

 notochoi'd-like structure in Annelids. 



27* 



