CONTRIBUTIONS TO THE EMBRYOLOGY OF NEMERTBA. 439 
from the archenteron (enterocel), but is nothing else than 
the original segmentation cavity, since it can be directly 
traced back as far as the blastula of figs. 1—38. This cavity is 
generally called—at least in the early stages—the blastoceel. 
I think it will be advisable and will prevent misunderstanding 
to limit the use of that name to the very earliest developmental 
stages, and to give to such cavities in the body of the larva and 
of the adult as can be demonstrated directly to arise out of 
this segmentation cavity a separate name, for which I would 
propose that of archicel. It indicates that such a cavity has 
indeed a very primitive significance, and must be held distinct 
from the archenteron and its derivatives.! 
The amceboid mesoblast cells, budded off both from epi- and 
from hypoblast thus are set free in the archiccel ; whilst they 
here accumulate in different regions and develope into different 
tissues, they do not fill up the whole of the cavity ; what 
remains of it may serve for different purposes, but in no case 
should its significance as part of the primary archiccel be 
overlooked. In the adult Nemertea it is the cavity of the 
proboscidian sheath, and, as we shall demonstrate further on, 
also the cavity of the blood-vascular system which must be re- 
garded as an unmistakeable Archicclom. 
Now, it may, perhaps, not always be an easy task to distin- 
guish an archiccelic cavity from a schizoccelic, especially in 
cases where the developing tissues bulge out and mask the 
archiceelic cavity temporarily from our view, so that this cavity 
only reappears later on, and is then liable to be looked upon as 
appearing for the first time as an effect of a special cause. We 
must well keep in mind that these two phenomena are essentially 
different, and that the continuity of the original segmentation 
cavity, whether temporarily invisible or not, is a passive pheno- 
menon, for which no further adaptative and hereditary processes 
have to be invoked, whereas the appearance of a cavity by actual 
1 Claus (‘ Typenlehre, 1874) und Hatschek (‘Entwgesch. der Anneliden,’ 
1878) have already called attention to the significance of this kind of primary 
body cavity. Hatschek even distinguishes a group of Vermes archicelo- 
mata (I. ¢.). 
