42 Ad EM [VOL Ln 
that dies first, therefore, consists of the two quarters 2.4.x. and 
7.p.z., and with their disappearance the remaining quarters are 
thrown toward the median line in such a way that the position 
of each organ is shifted a step farther toward the median line 
than the corresponding organ just behind it. Furthermore a 
row of dissimilar, unpaired organs is formed along the median 
line in the same sequence from behind forwards, as from the 
median line toward the sides, Fig. 2, @, 0, c, d, 2, and A, B, 
CD ae 
The steps in this process are as follows. The most median 
organs, a@ and a in the anterior row, Fig. 1, fuse with each other 
in the median line and then disappear. Their place is imme- 
diately occupied by the fused organs that were originally next 
to them on the outer side, namely 4 and 4; at the same time 
a and a fuse in the second row. In the third step, the fused 
organs 4 and 6 of the first row disappear and the fused organs 
c and c take their place ; 6 and @ take the place of a+a in 
the second row, and a and a fuse in the third. The following 
steps are of the same nature and are repeated till the condition 
like that in the anterior part of Fig. 2 is reached. It may be 
continued still farther till all the organs have disappeared, the 
last fused organs to disappear being the two most lateral ones 
of the, first line:! 
In reality, however, no such condition as the one just 
described would ever be completely realized, owing to the 
presence at the posterior growing end of the body of groups of 
segments in which the organs are arranged in an inverse manner 
to that just described, see lower half of Fig. 2. The tendency 
will be, therefore, to produce a body in which the arrangement of 
the organs is determined by the interaction of growth and 
degeneration, one having its greatest activity at the pos- 
terior, the other at the anterior, end. The resultant form 
will be therefore a more or less elongated rhomboid. The 
tendency of all segmented animals to assume such a form, 
1 It is not claimed that the process of median fusion and degeneration always 
follows exactly the steps indicated above, as will be observed on examination of 
the surface views of various embryos, but the variations from it are not of such 
a nature as to invalidate the general law. 
