ON THE ANCESTRAL FORM OP THE CHORDATA. 359 



considered as secondary, the hypoplastic origin as the primary, 

 or ancestral arrangement), it cannot be considered as absolutely 

 necessary, that in the other offshoot, the Nemertines, the hypo- 

 blastic origin of the proboscidian sheath should first be demon- 

 strated before any homology between notochord and proboscidian 

 sheath may be accepted. In Nemertines as Avell as in Verte- 

 brata the mesoblastie origin of the proboscidian sheath might 

 be a secondary condition. In that case much value could not 

 be attributed to special cases of coincidence in the embryolo- 

 gical data, and it would be the most primitive representatives 

 of both groups which would more especially be likely to fur- 

 nish evidence of a conclusive character. However, we must 

 here wait for more circumstantial evidence before being justi- 

 fied in further advancing in the domain of speculation. Yet, 

 with respect to this question, it must not be overlooked that 

 the fact of the cavity of the proboscidian sheath carrying pecu- 

 liar corpuscles, and of its being a closed sac lined by an 

 epithelium, tends far to give it the general character of a 

 body-cavity, a coelomic diverticulum, which is indeed situated 

 dorsally and longitudinally, but which by those general cha- 

 racters would lead us to expect a derivation from the archen- 

 teron, rather than a schizocoelic origin in mesoblastie tissues. 



If we were inclined to accept the mesoblastie derivation as 

 primary, and wished to picture to ourselves a possible common 

 origin of notochord and proboscidian sheath in the common an- 

 cestor of Vertebrates and Nemertines, we should have to postu- 

 late as a.still more primitive arrangement an axial thickening 

 of the mesoblastie tissues, which became more solid in the one 

 and hollowed out by the proboscis in the other. This would 

 clash both with the hypoblastic origin of the vertebrate noto- 

 chord and with the phylogenetic significance of the hypo- 

 physis. 



We have now to consider certain aspects of the suggested 

 homology between notochord and proboscidian sheath. 



There is no doubt that the fully developed notochord of a 

 Vertebrate is a structure of an entirely different character from 

 the proboscidian sheath of a Nemertean. The one is a solid. 



