CONTRIBUTIONS TO THE EMBRYOLOGY OF NEMERTEA. 437 
tively late in making their appearance, and the proboscis of 
which we noticed the first origin has long penetrated into the 
blastoccel, has even become attached by embryonic muscle-cells 
(figs. 9 and 13) to the developing muscular body wall before 
its sheath has become a distinct layer. And even when this 
sheath has appeared it is first only one cell layer thick (figs. 
12—14), and shuts off a cavity round the proboscis from the 
rest of the blastoceel. Before this, however, the muscular 
coat of the proboscis itself has come into existence. We 
have seen that the inner cellular lining of the proboscis is 
directly derived from the primitive epiblast. The further de- 
velopmental phases of this epithelium have been repeatedly 
observed by myself, and at the same time the process by 
which the muscular coats that ensheath this internal epithe- 
lium, and that give to the organ its peculiar mobility and 
retractility, gradually develope out of mesoblast cells that apply 
themselves against the epiblastic invagination as soon as it 
grows backwards into the blastoceel to form the first trace of 
the proboscis. Elsewhere this process has been more elabo- 
rately figured (1. c. (30), pl. iv, figs. 58 and 59, 66; pl. v, 
fig. 81); it is diagrammatically represented in figs. 9, 11, and 
13. These figures may at the same time elucidate how the 
development of the nervous tissue in the proboscis goes hand 
in hand with that in the head, and continually remains in con- 
nection with the central lobe, whatever further differentiation 
may go on in these particular mesoblast-cell groups, out of 
which both the central nervous system and the muscles and 
nerves of the proboscis take their origin. 
The different facts here mentioned about the growth of pro- 
boscis and proboscidian sheath are first of all observed in the 
prostomial region, and here the unicellular wall of the embry- 
onic proboscidian sheath fuses with the developing muscula- 
ture of the head just in front of the brain-lobes. However, it 
is then not yet present in the metastomial region, where the 
muscular extremity of the proboscis was already noticed by us 
as fusing with the muscular body wall (fig. 13), before the 
sheath has appeared. This happens nearly simultaneously with 
