352 A. A. W. HUBRECHT 



(this, according to Dohrn's .interesting researches, is the case 

 in one of the lowest of the vertebrate scale — Petromyzon), 

 this invagination at the same time being directed 

 towards the anterior termination of the notochord, 

 and lying in its direct continuation (figs. 1 and 2), or 

 (as is the case in the higher Vertebrates) not directly on the 

 outer surface, but on that portion of the epiblast which has 

 become the stomodseum (fig, 6). In the latter case it arises 

 as a median dorsal outgrowth from the mouth-cavity, directed 

 towards that portion of the under surface of the brain where, 

 between Prosencephalon and Metencephalon, the infundi- 

 bulum travels downwards, this being at the same 

 time the limit up to which the notochord extends 

 forwards under the brain. The fact that an outgrowth 

 from the brain thus grows downwards to meet this epi- 

 blastic invagination sufficiently indicates that in ancestral 

 generations, where the hypophysis was a less rudimentary 

 orffan. some sort of connection existed between it and the 

 cerebral thickening of the central nervous system. 



The constant presence in all Vertebrates of an organ so rudi- 

 mentary as the hypophysis, and about the significance of which 

 no plausible explanation has as yet been offered, has already 

 been insisted upon above. 



Both facts are in favour of regarding it as a very ancient 

 structure, which was once of great importance, and had a 

 different and at the same time a more definite physiological 

 value. 



In tracing this ancestral significance, the relation to the brain 

 and the somewhat less direct but, nevertheless, unmistakable 

 relation to the notochord must not be lost sight of. 



We will now consider the ontogenetic and phylogenetic his- 

 tory of the Nemertine proboscis. In the lower Platyelminths the 

 researches of v. Graff, lately crowned by his brilliant mono- 

 graph, have brought to light the different stages through 

 which retractility of a portion of the tactile anterior extremity 

 of the body, in which urticating elements are present, leads 

 to the appearance of a definite proboscidian structure, which 



