40 PATTEN. [VoL. XII. 
right nerve-cord, s, which from its shape and position appears 
more intimately related to the third left chelicera than to 
any of the others. What appears to be the neuromere of 
the fourth right chelicera is small and triangular, and pushed 
to one side by the growth of the neuromere just in front of it. 
That this growth affects the whole right half of the segment is 
shown by the presence there of two rudimentary lateral eyes, Ze. 
IV. Fusion oF RIGHT AND LEFT HALVES OF THE EMBRYO 
AND ANTERO—POSTERIOR DEGENERATION. 
This remarkable phenomenon has been observed in so many 
different stages, that there can be no doubt as to the manner 
in which it takes place. Median fusion and degeneration 
begins at the anterior end of the embryo (except in the hour- 
glass type), and gradually extends toward the posterior end. 
In the typical cases, each organ in one half of the embryo 
unites with its fellow of the opposite side to form a median 
unpaired organ. Those nearest the median line unite first, 
and then degenerate, and those lateral to them follow in the 
order of their position, till the whole of the segment has 
disappeared. The steps in the process are most clearly shown 
by the appendages, the dorsal organs, and the nerve-cords. 
The other paired organs probably fuse and degenerate in the 
same manner, but owing to their indistinctness in surface 
views, it is not so easy to follow in detail their successive 
modifications. In this way A-shaped embryos are produced, 
showing various stages in the progress of the degeneration 
from the anterior toward the posterior end. 
Toward the close of the process we may find either a median 
row of papillae, representing two or three pairs of medianly 
fused appendages, Pl. VI, Fig 50, or an exhausted mass of cells 
at what was the posterior end of the embryo; and finally these 
useless remnants may in their turn disappear. The principle 
involved in the process is well illustrated by the adjacent Figs. 
Paani. 92: 
In Fig. 1, each square represents a segmental organ of some 
kind. The lower half of the diagram shows how the organs 
