No. I.] VARIATIONS IN LIMULUS POLYPHEMUS. 125 
segments, it is possible that the appendages on each side of this constriction 
belong to the third and fourth segments. 
FIG. 57, X 60, not sectioned. This is an embryo in stage C that has under- 
gone partial median fusion, accompanied by transverse constriction. It could not 
be determined which was the anterior end of the embryo. It has been placed in 
its present position on account of the hood-like fold at what is now the anterior 
end, as it resembles a fold that sometimes covers the cephalic lobes of other 
embryos. At the anterior end of the embryo are two pairs of appendages nearly 
fused. Back of them is a thin layer of undifferentiated ectoderm with a thick 
mass of underlying mesoderm, the latter causing the dark color of the prepara- 
tion. There is a longitudinal fold of ectoderm, probably the displaced marginal 
fold, on the left side of this area, and in about the middle of the same is a conical 
invagination, 2.a. 
At the posterior end, the fold extends around a second depression, from which 
arises a large protuberance, evidently formed by the fusion of a pair of appen- 
dages. There is no trace of a nervous system in this embryo, or any distinguish- 
able lateral boundaries to the mesodermic area. 
Fic. 58, X 60, sectioned. Here we have a thick, oval mesodermic area, from 
whose inner surface several pseudopodia-like bands of nuclei extend vertically 
into the yolk. There are no appendages or nerve-cords, but there is an axial, 
ectodermic thickening, which, at what we may call the anterior end, forms a large 
pyramidal elevation, containing a large number of degenerating nuclei. This 
elevation probably represents one or more pairs of fused appendages. At the 
posterior end, the axial thickening gradually culminates in a tub-shaped elevation 
with a slit-like depression on its upper surface. At the bottom of the slit is a small 
circular depression. Underlying the whole is a great mass of mesoderm cells. 
FIG. 59, X 60, sectioned. In this embryo the cephalic lobes are hardly distin- 
guishable. There are three small appendages on the right and four on the left 
side. The identification of the appendages can be approximately determined by 
the presence on the right of the dorsal organ. There is a great depression across 
the posterior thoracic and abdominal region, and below its thickened ectodermic 
floor, and apparently arising from it, are a great many free-yolk cells. 
The mesodermic area is nearly circular. Along the whole extent of its 
thickened margin arises a cloud of isolated cells that extends deeply into the 
yolk. 
The concrescing posterior margins of the mesodermic areas are very beautifully 
shown. The median limbs, c.m.a, are the most conspicuous in surface views. 
Sections show that this is due to the presence of a ridge-like thickening of the 
ectoderm, continuous with a compact band of underlylng mesoderm. 
On the peripheral margin of the mesodermic area the ectoderm is thin, and the 
underlying mesoderm is composed of isolated, lymphoid cells. The dark spot on 
the right, where the third appendage should be, is due in part to a small protuber- 
ance there, but in the main to the presence, below the surface, of an oval sac with 
clear-cut walls. It has the appearance of an enlarged mesoblastic somite. 
Fic. 60, X 60, sectioned. This is probably a very old embryo, for there are 
thick layers of chiten over the tips of the remaining appendages, such as is only 
seen in embryos ready to hatch. It at first seemed probable that the small 
depression, #/., was the remnant of the cephalic lobes. But on sectioning the 
embryo, the thickenings at what is now the upper end seemed to be, without much 
