ON THE ANCESTRAL FORM OP THE CHOEDATA. 351 



invertebrate animals we do indeecLfind an organ which, in my 

 estimation, ranks equal to the vertebrate notochord, and thus 

 supplies the much-desired transitional form by which the Chor- 

 data are allied to the lower Metazoa, and in fact to such forms 

 as have neither the much specialised organisation of the seg- 

 mented animals (Arthropods and Annelids), nor require to be 

 turned upside down before their homology with the lowest 

 Vertebrata is admissible. 



That I venture to state this hypothesis before I am able 

 to bring into the field a well-arranged host of facts in its sup- 

 port, must be ascribed to my desire to induce such fellow- 

 workers in biology as have more leisure and better occasion for 

 investigating the numerous problems it suggests than I have, 

 into taking up a question which cannot be but looked upon as 

 of the highest significance for modern morphology. 



According to my opinion the proboscis of the Ne- 

 merteans, which arises as an in vagi n able structure 

 (entirely derived, both phylo- and ontogenetically, 

 from the epiblast), and which passes through a part 

 of the cerebral ganglion, is homologous with the rudi- 

 mentary organ which is found in the whole series of 

 Vertebrateswithoutexceptio n — t he hypophysis cere- 

 bri. The proboscidian sheath of the Nemerteans is 

 comparable in situation (and development?) with the 

 chorda dorsalis of Vertebrates. 



After this brief statement of my hypothesis I will enter into 

 a short discussion of its different details. 



It is not my intention to consider the numerous modifications 

 of structure which the hypophysis cerebri presents in different 

 adult Vertebrates, nor its glandular appearance, the con- 

 nection into which it enters with blood-vessels,. &c. ; but I 

 wish to restrict myself to the comparison of its very first onto- 

 genetic stages, in which it may be presumed most purely to 

 reproduce its ancestral character. 



We find the hypophysis originating as an invagination from 

 the epiblast, arising either independently on the outer surface 



