420 A. A. W. HUBRECHT. 
proboscis arose, not as an inward growth of the secondary 
epiblast, but actually as an independent delamination in the 
primary epiblast. 
We have now to note that the delamination process by 
which we have just seen that both the dorsal plate of secondary 
epiblast and the proboscis come into existence, is of a still 
greater extension, and that active delamination with a different 
purpose takes place not only in the epiblast but also in the 
hypoblast, even at still earlier stages of development. The 
delamination to which I now allude might more adequately 
be termed a budding of cells, the latter not forming a con- 
tinuous “lamina.” This budding process, by which indepen- 
dent cells are set free into the segmentation cavity—and which 
is no other than the formation of the mesoblast—commences 
in the early gastrula stage, and probably continues up to the 
time that the discs of secondary epiblast have coalesced to form 
the definite integument of the young worm. It was repeatedly 
observed in all its different phases, and fully figured (30) (pl. i, 
figs. 12—20; pl. ii, figs. 23—26, 33; pl. ii). There is no 
doubt that it affects both the epiblast and the hypo- 
blast, and, secondly, that it is not definitely localised 
This process of proliferation, at the time it occurs, temporarily 
doubles those cells of the primary epiblast which undergo this 
transverse division ; the epiblast, at a later period (fig. 2), is 
again only one layer thick (figs. 6—11). The hypoblast is 
only in its very earliest phases a distinct unicellular layer. 
Later on the cell walls become less distinct. Nevertheless, 
the budding of the hypoblastic mesoblast cells into the 
blastoccel was here traced quite as positively. 
1 In an exceedingly able and suggestive paper on the “ Development of 
Aulostoma gulo” (‘Arb. aus d. Zool. Zoot. Institut zu Wiirzburg,’ Bd. vii, 
p. 231), R. 8. Bergh has, in addition to most valuable contributions to our 
knowledge of the ontogeny of the Discophora, instituted certain comparisons 
between these and the Nemertea. These comparisons are chiefly based on 
embryological data, and would indeed be more plausible if Barrois’s account 
of the ontogenetic processes in Lineus could be relied upon. Now that 
closer investigations lead to the conclusion that the mesoblast in Lineus 
