No.1.] VARIATIONS IN LIMULUS POLYPHEMUS. 145 
In embryo C the same kind of fusion and degeneration as in embryo Z has 
occurred, but it has progressed still farther, for the dorsal organs have fused, and 
also the six pairs of appendages. At the central ends of all the embryos are paired 
and unpaired ridges, representing abdominal appendages. The dorso-ventral 
muscles are well developed, and may be seen radiating from the triangular, 
marginal fold to all three embryos. 
Fic. 104, X 33. This extraordinary embryo is a triple one like the preceding. 
By median fusion and antero-posterior degeneration, each embryo is reduced to a 
condition like that in embryo C, Fig. 103. The dorsal organs and the fifth and 
sixth thoracic appendages have fused in the median line. Everything anterior 
to the fourth or fifth segment has disappeared. There is a dark, central area, 
composed of thickened layers of mesoderm and ectoderm, that represents the 
anal plates of all three embryos. A series of two or three concentric ridges 
encircle it, representing the abdominal segments. Between embryos 4 and B, and 
A and C, are seen the concrescing margins of the mesodermic areas, divided into 
distinct mesoblastic segments. Nothing of the kind can be seen between embryos 
B and C. 
The three following embryos belong on the preceding plate, but could not con- 
veniently be placed there. 
FIG. 105, X 60, not sectioned. A very flat embryo with inconspicuous, mar- 
ginal folds. The right chelicera is absent. The embryo is remarkable for the 
large size of the mouth, and especially for the series of organs arranged along the 
median line behind the mouth, a-d. The first two are transverse depressions, and 
in the yolk beneath the second depression, as though arising from it, is a collec- 
tion of cells. The third depression is similar to the second, but with a thick lip, 
projecting forwards from its posterior border. The fourth is a conical, ectodermic 
elevation. The abdominal plate is thrown toward the right. The embryo con- 
veys the impression (more strongly than appears in the figure) that it is provided 
with a series of five mouth-like openings, arranged along the median line. 
Fic. 106, X 60, sectioned. This embryo is in an advanced stage of develop- 
ment, but extensive degeneration has taken place, leaving but little of the original 
embryo behind. The cephalic lobes are represented by a dark patch of cells, 
with a still darker portion, the remnant of the oesophagus, in its centre. In 
sections, it appears as a slightly thickened layer of ectoderm, with a much thicker 
mass of mesoderm beneath it. The remainder of the embryo, except the terminal 
lobe, consists of a single layer of flattened ectoderm cells, with a thin underlying 
layer of mesoderm. There is no indication of a nervous system, or other 
ectodermic structures. The margins of the mesodermic area, especially on the 
side, are thickened, #.a., to form a conspicuous band, composed of several layers 
of cells. At the posterior end of the body of the embryo is a median, conical 
projection, probably representing the tip of the abdomen, but possibly a fused 
pair of thoracic appendages. The appendage rises out of a depression, the 
anterior wall of which is thrown forward into a shallow pocket. 
Back of this papilla, the margins of the mesodermic area have concresced in 
the typical manner for stage D, and in the yolk beneath the concresced margins 
is an oval cloud of degenerating cells, Z.a.c. The condition of the posterior end 
of the embryo shows clearly, in spite of its apparently simple condition, the ad- 
vanced stage of development it has reached. While degeneration seems to be 
