No. I.] VARIATIONS IN LIMULUS POLYPHEMUS. A 
embryo lies. Its thickened rim, 7. a., usually forms two con- 
spicuous posteriorly directed loops, which may be found in all 
stages of concrescence behind the apex of the abdomen. The 
sides of the loops concresce like the closing of the arms of a \, 
and a longitudinal post-anal thickening is thus formed, below 
which lies a great cloud of cells, brought together at that point 
by the union of the proliferating rim. 
In the abnormal and degenerate forms, the margin of the 
mesodermic area is usually very distinct, although every out- 
ward trace of segmentation in the mesoderm, and even the 
greater part of the embryo itself, may have disappeared. 
The margin also shows zz many cases more clearly than in the 
normal embryos the posterior, median concrescence, and it takes 
place in such a manner as to bring very forcibly to mind the 
similar phenomena in vertebrate embryos. One of the most 
striking instances of this exaggeration of the mesodermic 
margin is shown in Pl. VI, Fig.63. Unlike all the rest, this 
is an opaque embryo shown by reflected light. The margin of 
the mesodermic area forms a conspicuous ridge, which is 
thickened posteriorly. Where concrescence has taken place, a 
broad median elevation is produced. 
Nothing like the thickened margin here described is known 
to occur in any other arthropod, but a similar concrescence of 
mesodermic segments probably occurs in all arthropods. 
In the embryos very far advanced in degeneration, the margin 
of the mesodermic area breaks up into isolated, irregular masses 
containing closely packed nuclei, Pls. VI and VII, Figs. 60, 
72, 76, 88, etc. These masses may lie deeply in the yolk, 
but they still show very clearly their derivation. They fre- 
quently send out pseudopodia-like streamers of nuclei, which 
fade at the distal ends as though they were gradually dissolving 
in the yolk. 
In Figs. 67 and 72, the margins of the mesodermic area first 
united a short distance behind the end of the abdomen, leaving 
a pear-shaped area of yolk, covered by the blastoderm only. 
In Figs. 10, 18, 24, 61, 68, the margins have fused over a 
long distance, leaving along the line of fusion a cloud of yolk 
and mesoderm cells. The posterior margin of the fused areas 
