No. I.] VARIATIONS 1N LIMULUS POLYPHEMUS. 61 
end of the thorax, and at the anterior end of the abdomen. 
Each of the regions between these points may degenerate inde- 
pendently of the other, and in the same way that the whole 
embryo sometimes does, suggesting the idea that the embryo 
consists of a chain of tmperfect individuals, such as we see 
in annelids as the result of tmperfect fission. 
A less strongly marked division occurs in Limulus, across the 
middle of the thorax between the third and fourth segments. 
It is shown (1) by the marked tendency of the first three seg- 
ments to degenerate and of the last three to persist, (2) by 
the transverse fission that occurs there, see following para- 
graphs (p. 67), and (3) by the presence of an enlarged pair of 
sense-organs (“dorsal organs’’) opposite the first of the last 
three segments, just as the lateral eyes lie opposite the first 
one of the first three segments. 
When the degeneration is incomplete, it may manifest itself 
by a partial median fusion and a decrease in size of the organs 
along the lines separating the regions above indicated. 
These cross-lines, where degeneration ts most likely to occur in 
Limulus embryos, are the regions showing the least “vitality’’ ; 
they correspond to the regions where a reduction in size and a 
tendency to undergo median concrescence are frequently seen 
in the adult individuals from various groups of arthropods. 
In Limulus these lines of weakness are cleavage planes separat- 
ing regions having different potentialities, and corresponding to 
regions possessing different morphological characters in many 
other arthropods. Broadly speaking, the salient morphological 
characters seen in the various regions of the body of arthropods 
are similar to those that appear as abnormal variations in Limulus 
embryos ; and, as we have seen, these characters are mainly those 
due to difference in size, specialization, and median fuston, and 
these tn turn have to be referred back primarily to differences im 
the power of growth. 
The four regions of the body in Limulus embryos are 
roughly shown in the adjacent Fig.6. The lateral constrictions 
and the transverse lines indicate approximately the position 
and amount of median fusion and degeneration. 
The repetition of three segments in successive regions of 
