VARIATIONS IN LIMULUS POLYPHEMUS. 105 
EXPLANATION OF PLATE III. 
Fic. 8, X 60. This is a well-developed and nearly full-sized embryo of stage 
C-D. 
It is remarkable on account of the invagination of all the thoracic appendages 
except the first and last pairs. The first pair are small for this stage, and hardly 
recognizable even in sections. The last pair are well developed, and project freely 
from the surface in the normal way. 
The invagination of the remaining thoracic appendages is greatest on the left 
side. The series of low oval elevations represents the uninvaginated bases of the 
appendages, and the transverse slits at their summits, w.af., the openings leading 
into deep cavities formed by the invaginations of the outer two-thirds of the 
same. The dark ring around the slits represents the optical sections of the 
ectoderm nuclei that form the wall of the invagination. Sections of these 
appendages show that they do not differ materially from those shown in Pl. XI, 
Fig. I. 
The abdominal plate is very short and devoid of appendages. A small, deep 
pit is present in it, which in sections resembles the proctodaeum. Comparison 
with stages C and D indicates that the embryo is too young to be normally pro- 
vided with a proctodaeum. It probably represents the last stages of the se/opore. 
The cephalic lobes are abnormal in shape and structure. They contain two 
central depressed areas of thickened ectoderm, appearing as light round spots in 
surface views, 47.zv, and in section, Pl. XI, Fig. 8. The optic ganglia, of.g., are 
partly concealed by a broad fold of ectoderm, whose free edge is directed diagonally 
backward, g.f, Pl. XI, Figs. 8 and 8%. This fold does not extend across the 
median line in front of the oesophagus. 
The lateral eyes are conspicuous, but the sense-organs opposite the third 
thoracic appendage are not visible in surface views, although they can easily be 
detected in sections. 
Fic. 9, X 60. This embryo is between stages C and D. The anterior end of 
the body is narrowed, and there is a slight lateral constriction opposite the fourth 
pair of appendages. The latter are completely invaginated into the yolk, leaving 
two slit-like openings on the surface of the embryo. The boundaries of the inner 
ends of the invaginated appendages are not sharply defined, and in the yolk 
around them is a halo of degenerating nuclei apparently formed by the disintegra- 
tion of its walls. The thoracic portion of the marginal fold is very faint, except at 
the posterior end. The whole embryo lies flush with the surrounding surfaces, 
instead of being deeply depressed as in Figs. 8, Io, 12. 
The cephalic lobes are clearly outlined. A series of transverse sections (Pl. XI, 
Fig. 9) shows that the dark area bounded by the lines ec. and gf is covered by an 
amnion-like, ectodermic fold (although the middle layer is very obscure), extending 
backward and medianly over the cephalic lobes. Over the light inclosed area, 6r., 
there is no superficial ectoderm, and the very light spots, 47.zv., are the same pit- 
like depressions of the brain seen in Figs.8-12. The ectoderm over these depres- 
sions is covered with a layer of vertically striated, cuticular substance, looking like 
a layer of cilia, and similar to that seen over the surface of most developing sense- 
organs. 
