358 A. A. W. TIUBKECHT 



has postponed giving the details of this connection to a hiter 

 publication we cannot at present judge of its significance. 



Hoffman is the only other author who gives any details 

 about the formation of the proboscidian sheath. According to 

 his account of sections made of Tetrastemma, a portion of the 

 proboscis is split off from the dorsal surface of the ali- 

 mentary canal. The muscular proboscidian sheath is meso- 

 blastic in origin. This observation, which can hardly be 

 brought to agree with the epiblastic origin of the proboscis, 

 noticed above, might perhaps allow of a different interpreta- 

 tion. As a simple suggestion I would advance, that perhaps 

 Hoffmann may have mistaken the formation of the inner por- 

 tion of the proboscidian sheath (so often confused with the 

 proboscis !) for that of a part of the proboscis itself. 



Hypoblastic formative elements internally would then coa- 

 lesce with raesoblastic derivates, more especially muscular 

 elements, exteriorly applied to the former and constituting 

 together the proboscidian sheath, i.e. the wall of the probos- 

 cidian chamber. 



Such an interpretation would appear to be more acceptable 

 than the coalescence of a tubiform derivate of the hypoblast, 

 with an invagination of the epiblast travelling backwards, the 

 fusion of these two giving rise to the definite, cylindrical, evers- 

 ible proboscis. Balfour, in his * Comparative Embryology,' is 

 not inclined to accept Hoffmann's statements without further, 

 confirmation. 



Still this observation, if it may be interpreted as proposed, 

 would be of importance and its repetition much to be desired. 

 This and Barrois' description above cited appear to open the 

 })rospect that embryology may eventually succeed in 

 demonstrating for the proboscidian sheath, or for one 

 of its constituent parts, a hypoblastic origin. 



If such be proved to be the case, not only its situa- 

 tions but also its development would correspond to 

 that of the notqchord of the lo wer Vertebrates. Still, 

 considering in how many cases the origin of the notochord in 

 Vertebrates is apparently mesoblastic (this phenomenon being 



