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DEVELOPMEN'T OF BALANOGLOSSUS KOWALEVSKII. 515 
amongst the mesoblastic tissues. The question as to the 
nature of these fibres is one of great interest. They may 
either be mesoblastic fibres penetrating into the ectoderm as 
supporting structures, or they may be epiblastic fibres leaving 
the skin, in which latter case they are in all probability 
nervous. 
Somewhat similar fibres have been described by Ludwig in 
the similar tissues of Asterias, and he is of opinion that they 
are connective tissue. The possibility, however, that these 
fibres in Balanoglossus are nervous is supported—firstly, by 
the fact that they always taper inwards and not out- 
wards ; secondly, that as a matter of fact, in B. Robinii at 
all events, the ectoderm cells may themselves be traced into 
tails of this kind; thirdly, the general absence of nuclei in 
the ‘‘ punktsubstanz,” for if these fibres are supporting cells, 
nuclei might be expected to be found in their course ; fourthly, 
there is an 4 priori difficulty as to the nerve supply to the 
muscles in these animals, for, though the body of some of the 
species is very thick, no definite nerve-cords are to be 
found crossing the body cavities, with the exception of the 
*‘ dorsal roots’? mentioned hereafter. How, then, are the 
muscles innervated? It seems, then, at least possible that 
the nerve supply is derived directly from the skin, in which 
case the fibres leaving the “ punktsubstanz ” naturally suggest 
themselves as the transmitting agents. Finally, the view that 
these fibres are ectodermic is rendered likely from the fact 
that their origin may occasionally be traced from a very high 
level in the skin, though the appearance which is sometimes 
produced in sections as of their actual continuity with the 
undoubted ectoderm cells may not be quite reliable. In a 
few instances these fibres appear to anastomose with meso- 
blastic elements, though this cannot be quite definitely 
affirmed. On the whole, the balance of evidence seems in 
favour of the view that they are ectodermic. If this be correct 
the skin of Balanoglossus is to be regarded as a collection of 
sensory cells ending in long fibres, which may either be con- 
nected to the central nervous system, probably by the longi- 
