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That is to say, not the histological structure, but the manner 
of its development makes the chorda to be the chorda. Have 
we then any knowledge of development at all, which is not 
that of one histological structure in the midst of other histo- 
logical structures? But then the chorda is nothing more 
than “a connecting-piece ” between the two symmetrical 
halves of the vertebral system. We are really compelled to 
ask what is it that is actually, and in what way, con- 
nected? The vertebral system grows long after the origin 
of the chorda, from both sides over the chorda, encloses it, 
and finally produces the result that the chorda entirely dis- 
appears. What becomes then of the function of the “ con- 
necting-piece?” And further, the chorda is asserted to be 
primarily merely “the connecting-piece of the two sym- 
metrical halves of the vertebral system, although at the 
same time it is itself a part of the vertebral system.” 
According to the first part of this definition, the chorda 
cannot be an integral part of the vertebral system; for then 
it could not connect the two halves of this system. Accord- 
ing to the second part, however, it is an integral part of 
the same, and, at the same time, its proper connecting- 
piece. Which half of this definition are we to believe? 
Does the truth, perhaps, lie there also in the middle, and 
connect the two symmetrical halves of the definition in the 
same way as the chorda connects those of the vertebral 
system? In reality the definition of Dr. Donitz is nothing 
more than a vague dogma, which has not the least connective 
force in it. In direct opposition to this dogma, the chorda 
appears really to have nothing at all to do with the vertebral 
system, but is originally, in all probability, only a connective- 
tissue rod to which a portion of the body-musculature has 
become attached. Through this its gradual change into hy- 
aline cartilaginoid substance is brought about, as we remark 
everywhere in the embryos of vertebrata, as to the change of 
connective-tissue into cartilage. Finally, as quite recently 
has been especially proved by Wilhelm Miiller, the con- 
nective tissue in the neighbourhood of the chorda grows out 
into a skeleton-forming sheath, and becomes ossified. And 
accordingly, since the musculature was previously segmentally 
arranged, the skeleton-forming sheath becomes also segment- 
ally arranged in connection with it, and receives the verte- 
brate character. And so, as a result, the chorda has nothing 
to do with the vertebre, but is, on the contrary, gradually 
squeezed out of existence by them. 
As to Dr. Donitz’s opinion, therefore, that the development 
of the Ascidians speaks in a most emphatic manner against 
