THE ANCESTRY OF THE CHORDATA. 549 
answer to this question can only come in a fuller understand- 
ing of the laws of growth and of variation which are as yet 
merely terms. 
Preliminary Remarks onthe Repetition of Organs 
of the Chordata. 
In the foregoing pages it has been attempted to show (1) 
that repetition of organs and sets of organs is of common 
occurrence among animals, and (2) that however far back a 
segmented ancestor of a segmented descendant may possibly 
be found, yet ultimately the form has still to be sought for in 
which these repetitions had their origin. Hence it follows 
that in no case must it be held & priori impossible that an 
unsegmented form showing no degeneration shouid be related 
to a segmented stock. But when inquiry is made in the 
special case of the Chordata as to the condition of the repe- 
titions found among them, it will be seen that so far are they 
from suggesting that their immediate ancestor of the group 
must have been segmented, that they even preclude this view. 
As will be shown, there is a history of the actual steps by 
which several of the organs (the nervous system, the axial 
skeleton, and the mesoblast) acquired their repetitions within 
the group, and certain other structures (the notochord, &c.) 
persist in an unsegmented form. So that instead of regarding 
a fully segmented form as their possible ancestor it is neces- 
sary to search for a form in which these particular sets of 
structures at least are not repeated. 
For in the first place, taken generally, the development of a 
Vertebrate consists in the gradual appearance of repetitions, 
first of one organ and then of another, until at last a climax is 
reached. The mesoblast divides into blocks, paired peripheral 
nerves grow out, and segmented tubules arise in connection 
with the excretory ducts, but the mesoblastic plates were at first 
unbroken, the medullary plate continues without transverse 
divisions, though its peripheral organs may be repeated, and 
the excretory ducts are single tubes with single openings. That 
many of these structures roughly correspond with each other 
