436 A. A. W. HUBRECHT. 
very soon begin to accumulate against the internal surface 
(figs. 5, 6, and 11). I expect that it is this accumulation 
which has also been observed by Barrois, and which has been 
very differently interpreted by him, viz. as a proliferation of 
the plates of secondary epiblast from which the mesoblast 
originated. Our actual sections have, however, sufficiently 
demonstrated that it is not a proliferation but an accumula- 
tion of mesoblast cells which was observed, and that the 
latter originate in a different way, not from the secondary but 
from the primary epiblast. 
It can without difficulty be demonstrated that the outer 
layer of longitudinal fibres developes at an early stage out 
of the mesoblastic material accumulated against the secondary 
epiblast, and that the latter remains a unicellular layer for a 
comparatively long period, long after the larva has been set 
free. The appearance of the outer longitudinal muscular 
layer is, as we have already noticed above, simultaneous with 
that of the longitudinal nerve-stems, as is that of the brain- 
lobes with that of the muscular tissue in the head. The details 
of the transformation of embryonic into muscle-cells have been 
figured elsewhere (1. c. (30), pl. iv, figs. 64—71). 
It is only much later that the circular muscular layer and 
the inner longitudinal layer make their appearance, so as to be 
clearly distinguishable (1. c. (30), figs. 69, 70). The larva has 
then long ago shed the covering of primary epiblast, and moves 
about in the gelatinous strings by which the egg-capsules were 
enclosed when they were deposited by the animal, and which 
now appears to serve as a pabulum for the young larve. That 
the development of these two muscular layers is indeed of com- 
paratively late occurrence can be more especially demonstrated 
in the dorso-median region, where there is present a longitu- 
dinal fold in the hypoblast (fig. 14). The space enclosed be- 
tween this fold and the developing body wall is nothing else 
than the original blastoccel, and in this dorso-median region 
must arise, in addition to the muscular layer just mentioned, 
the proboscidian sheath with its internal epithelial covering 
and its external musculature. Both of these are also compara- 
