CONTRIBUTIONS TO THE EMBRYOLOGY OF NEMERTEBA. 433 
have to be demonstrated ; the primary having been isolated from 
the tissues forming the rest of the body by the formation of the 
secondary epiblast in the way described above, and only con- 
tributing towards the formation of the sensory epithelium of 
the proboscis and of the posterior brain-lobes in the way 
traced in preceding pages. 
This fact must be remembered when we consider the 
apparently unexpected fact of the mesoblastic origin of the 
nervous system. A certain number of the mesoblast cells did 
arise out of the primary epiblast, and it is in no way improbable 
that these might in the first place contribute to the formation 
of the nervous system. If this could be proved it would cer- 
tainly not make such a very great difference, that the nerve- 
cells, instead of developing into the central nervous apparatus 
in loco, first changed their situation in accordance with the 
further changes in the primary epiblast and with its final 
rejection. The nervous plexus, which some years ago was 
demonstrated by me as being present just outside the layer of 
circular muscles, can be observed in early larval phases ; in very 
young animals, only a few millimetres in length, it is imdu- 
bitably present, and as the tissue composing it passes in the most 
gradual manner into that of the lateral nerve stems, I have 
no doubt that it developes in exactly the same way and at the 
same period, i.e. from the mesoblast cells just mentioned. 
The same must be admitted for the few separate and inde- 
pendent nerves, that have been observed in Lineus (as in other 
Schizonemertea) to emerge from the central apparatus, viz. 
the nerves to the tip of the snout, the nerves for the proboscis, 
and the so-called vagus nerve which springs from the lower 
brain-lobes and innervates the cesophagus. The mediodorsal 
nerve (so called proboscidian-sheath nerve) is no more than a 
local thickening in the cylindrical plexus just alluded to. In 
tracing the origin of the musculature of the proboscis we shall 
see that the development of its nerves in the way just indicated 
is not only probable and intelligible, but that this view may be 
said to be the only one that fits in naturally with the develop- 
ment of its musculature. 
