438 A. A. W. HUBRECHT. 
the appearance of circular and inner longitudinal muscular 
layers, but in the beginning no muscular elements are detec- 
table in the sheath, the cellular epithelium being the only 
representative of it, especially in the head. Only gradually 
muscular fibres develope outside of this epithelial lining, and 
these fibres again differentiate into different layers. There is 
thus absolutely no escape from the conclusion that the epithe- 
lial lining of the proboscidian sheath arises out of a laminar 
arrangement of mesoblast cells. So do the muscles of sheath 
and proboscis, as well as the free corpuscles floating in the 
fluid which is found between the proboscis and its sheath, 
and is of primary importance in the act of expulsion of the 
proboscis. 
We must now return to the cavity in which the proboscis 
moves. In the early stages this is no other than the free 
cavity between the body wall and the hypoblast, which has not 
arisen—as the numerous preparations clearly show—by a 
splitting of the mesoblast (schizoccel), nor by a differentiation 
1 It must here be observed that the results which I have obtained concern- 
ing the development of the proboscidian sheath apparently give no support to 
the hypothesis which I ventured to make a few years ago (‘ Quart. Journ. 
Mier. Sci.,’ vol. xxiii, 1883, p. 349), according to which if we regard the 
hypophysis of Vertebrates as a rudimentary proboscis of their Invertebrate 
ancestors, we might also compare the notochord to the proboscidian sheath. 
Amphioxus, developing its notochord out of a dorso-median furrow of the 
hypoblast, it would have been a most valuable argument for this hypothesis 
if also in Nemertea the proboscidian sheath arose in a similar way. This we 
have seen is not the case. We must, however, not forget that if any such 
positive argument is not furnished by the facts of ontogeny, neither is any 
argument contrary to that hypothesis implied in those facts. For if we 
remember that the dorso-median proboscidian sheath arises out of mesoblast 
cells, and that the mesoblast in part developes out of the hypoblast, it would 
not be impossible that, by the help of some new method of investigation 
which would allow us to follow the origin of the individual mesoblast cells, 
we might after all demonstrate that the mesoblast cells that become the pro- 
boscidian sheath are all hypoblastic and originate out of this primary layer in 
the regions where they are subsequently found, i.e. medio-dorsally. And in 
that case, which is in no way rendered improbable by these researches, the 
comparison between notochord and proboscidian sheath would receive very 
emphatic support. 
