CONTRIBUTIONS TO THE EMBRYOLOGY OF NEMERTEA. 455 
cells of the lobes then assume this particular appearance, and 
no derivation from cesophageal tissue, which I held to be not 
improbable in a former publication when I could only consider 
the embryological data as they were furnished by previous 
authors, can be any longer upheld. 
Of all authors who have given descriptions of the origin of 
the nervous system in Nemertea—and from all of whom my 
own results differ—few give such full details as Salensky in 
his latest article (27). Although he has examined a different 
species, belonging to a different group of Nemertea, in which 
development is direct and no Desor’s larva occurs, still I may 
be allowed to presume that on this head the observations of 
the Russian naturalist are less accurate than others we owe to 
his trained eye. In the Dutch version of my researches I 
have given a full account of the points of divergence between 
his results and mine, and have attempted to show that the 
figures which he gives, fit in much more naturally with my inter- 
pretation of the facts as I found them in Lineus than with his 
own, and make it highly probable that also in Amphiporus 
viviparus the nervous system has a mesoblastic origin. In 
that case we may at the same time leave Salensky’s sugges- 
tion—which, however, he has far from proved—viz. that the 
lateral nerve-cords arise as posterior outgrowths from the 
brain-lobes, out of further discussion, referring to the cited 
memoir. 
Further products of the mesoblast besides the nervous sys- 
tem are the muscular layers of the body wall, those of the 
proboscis and the proboscidian sheath, and the hyaline ground 
substance in which all these are embedded, and which wholly 
fills up the space between the intestinal, nephridial, generative 
and vascular cavities. I will now give a short account of the 
further development of these mesoblastic products, together 
with which will be discussed the origin of the cavities of the 
proboscidian sheath and of the blood vascular system. 
After the formation of the five plates of secondary epiblast, 
which are to furnish the epiderm of the adult, has commenced 
in the way above described, we saw that the mesoblast cells 
