No. 1.] VARIATIONS [IN LIMULUS POLYPHEMUS. 127 
Fic. 63, X 50, not sectioned. This embryo is shown as an opaque object by 
reflected light. It was drawn shortly after it was killed in picronitric acid, and 
while still in the fluid. 
The embryo is small, but has six pairs of thoracic appendages and well- 
developed cephalic lobes. It shows in a very striking manner a conspicuous ridge 
extending completely around the embryo. It is broadened and thickened poste- 
riorly, and is continuous, with a thick, broad elevation extending forwards to the 
posterior portion of the body. ‘This ridge is the very well-developed marginal 
thickening of the mesodermic area, such as has been shown in many of the preced- 
ing figures. The post-anal band is formed by the concrescence of the posterior 
margin of the mesodermic area. When seen in this way, as an opaque object, its 
resemblance to the blastodisc of a vertebrate embryo is obvious (shark, lizard, or 
chick). 
Fic. 64, X 60, not sectioned. A peculiar embryo without appendages, but 
showing three pairs of distinct, hollow, mesoblastic somites. In shape it resembles 
the embryos of scorpions or spiders more than those of Limulus. 
Fic. 65, X 60, sectioned. A small, abbreviated embryo in stage C. The 
rudimentary cephalic lobes are depressed and rotated to the embryo’s right. 
Their lateral margins are still further invaginated to form two oval depressions, 
nearly concealed by distinct folds that have grown over them from the sides, 4.7v. 
The oesophagus is comparatively large, and the slit-like opening is also rotated 
toward the left. 
The chelicerae are probably absent. On the left are the fourth (?) and sixth (?) 
appendages, but the fifth is invaginated. On the right the fourth (?) and fifth (?) 
are invaginated, and the sixth (?) persists. 
A telopore is present behind the last pair of appendages. Back of the embryo 
is seen a long, faint band of yolk cells, .¢.c., formed by the concrescence of the 
margin of the mesodermic area. 
Fic. 66, X 60, not sectioned. A small embryo in stage C. 
The cephalic lobes are not distinguishable as such, but the oesophagus is 
distinct. There are three appendages (fourth, fifth, sixth[?]) on the left, and 
two on the right. In place of the posterior part of the thorax is a broad 
depression. 
FIG. 67, X 60, not sectioned. A narrow embryo, depressed along the median 
line, and surrounded by high and sharp marginal folds. The cephalic lobes are 
depressed below the level of the thoracic nerve-cord. The second(?) and third(?) 
pairs of thoracic appendages are present with one more appendage on the 
right. In the abdominal region is a deep, pyramidal depression, directed nearly 
vertically downwards. 
The dark area along the marginal fold of the embryo is produced by unusually 
thick layers of mesoderm. The margin of the mesodermic area is not clearly 
defined ; it lies, in part, at least, beyond the limits of the figure. On the left side 
of the embryo, in the thickened mesodermic margin, is a large, marginal vesicle. 
The two dark areas back of the abdomen are formed by longitudinal, ridge-like 
thickenings of the ectoderm and mesoderm. Between the two ridges is a depressed 
area covered by a thin layer of mesoderm and ectoderm. It appears, therefore, 
that the posterior median margins of the mesodermic area have not completely 
united, except at their very posterior ends. In front of this point is a rather 
large rhomboidal space not covered by the mesodermic area. 
