VARIATIONS IN LIMULUS POLYPHEMUS. 109 
EXPLANATION OF PLATE IV. 
Figs. 16, 17, (18), 19, 20, 21, (22), 23, 24, 26, 27 represent very common types 
of about stage C. They are often laterally compressed, as well as diminished in 
length by the antero-posterior compression of the head region, and by the 
absence of the abdomen. Except in Fig. 19, the marginal fold, g.f, is very con- 
spicuous, extending nearly around the embryo. All the parts included in this 
fold, z.e. the nervous system and appendages, are deeply depressed below the 
surface of the ovum. In Fig. 19, there is a difference in direction between the 
appendages of the anterior and posterior thoracic regions, and a wide space 
between the third and fourth pairs, in Fig. 23. In Fig. 16, the second and third 
pairs of existing appendages are partly invaginated. There is no invagination of 
the abdominal plate in Figs. 17 and 19, but a very conspicuous one in Figs. 16, 
20, 21, and 23. The full number of thoracic appendages are present in Figs. 17, 
19, 20, 22, and 23. In Fig. 16, two metameres are absent, probably the first two, 
although there is no positive evidence in this case as to which ones. In all the 
figures the cephalic lobes, and the cheliceral segment when present, are much 
more deeply depressed than the rest of the nervous system. This is a very 
common feature in this type of embryo, and is especially well shown in Fig. 16, 
where there is a very abrupt descent from the level of the ventral nerve-cord to 
that of the cephalic lobes. Compare also Figs. 62-67, etc. 
Two extreme modifications of the ganglionic folds of the cephalic lobes are to 
be seen. One is shown in Fig. 19, where the cephalic lobes are nearly flat and 
naked, or as in this embryo, covered by a very thin single layer of cells, ¢.f, ad- 
vancing posteriorly and medianly over the lobes and adhering very closely to 
their outer surface. The other type is seen in Fig. 21, where there is a deep 
invagination on either side of the cephalic lobes, overarched by a very prominent, 
amnion-like ganglionic fold. In Figs. 17 and 23, only a’small part of them is so 
concealed, while in Fig. 16 they are entirely exposed. In Pl. IV, Fig. 34, the 
whole cephalic lobes are covered by this fold. 
In nearly all of these figures, the lateral eyes are very well developed for this» 
stage, and are easily seen in surface views about opposite the chelicerae, or if the 
chelicerae are absent (as in Fig. 16) about opposite the position the chelicerae 
would have occupied had they been present. On the other hand, the so-called 
“dorsal organs” are either entirely absent, or so faintly developed that they can- 
not be detected in sections or surface views. They are shown in Fig. 22 only. 
In these compressed embryos, the rim of the mesodermic area is generally very 
thick and distinct. In Fig. 20, the outlines of the posterior mesoblastic somite 
are visible up to the margin, and the manner in which they grow toward the 
median line, and unite there to form the post-anal cloud of cells, is clearly shown. 
In most embryos of this type, the outlines of the somites are not preserved, but 
the posterior portion of the thickened rim of the mesodermic area, and the part 
that has concresced in the posterior median line are all the more distinct. One 
of these forms, seen as an opaque object and illustrating the extreme development 
of this character, is shown in Pl. VI, Fig. 63. An important point to be observed 
here is that the rim of the mesodermic area is so abnormally large, that it appears 
as a white and prominent ridge, resembling in shape and position the so-called 
concrescing margin of the blastopore in sharks and reptiles. 
