Nas Fo] VARIATIONS IN LIMULUS POLYPHEMUS. 103 
zation and in general appearance resemble sense-organs. The suggestion that 
the appearance is due to a folding to increase the surface, or that they represent 
“neuroblasts” (Wheeler) is inadequate, and both suggestions are based on mis- 
apprehension of the facts. We shall treat of them more fully elsewhere. 
The rim of the mesodermic area is considerably enlarged, and is growing 
towards the median line back of the anal plate. 
The telopore in this particular individual has disappeared, but the median 
streak of inner layer cells, derived from it, is still visible. 
Fic. 5, X 60. Surface view of a normal embryo 18 (?) days old. All the 
thoracic appendages except the first pair are now elongated processes having the 
characteristic shape of thoracic appendages. The sixth pair are still transversely 
elongated, and resemble in form the early stages in the development of the 
abdominal appendages. The most striking feature of this stage is, however, the 
series of large, shallow depressions on the outer side of the marginal fold, m./f. 
It is only in exceptional cases that the depressions are visible. They are most 
readily seen by reflected light in shelled ova, shortly after treatment with the 
hardening reagent, picro-nitric acid. The second and third, and the fifth and 
sixth, quickly disappear, leaving no trace behind. The first is a little smaller 
than the rest and lies a little in front of the chelicerae, but appears to belong to 
that segment. It develops into the lateral eyes. The fourth, d.0., lies exactly 
opposite the fourth thoracic appendage, and growing rapidly, gives rise to the con- 
spicuous organ, undoubtedly of a sensory nature, found in nearly the same position 
as late as the time of casting the first larval shell. The lateral eyes gradually 
move backwards till they lie on the dorsal side of, and posterior to, the so-called 
dorsal organ, d.o. The rim of the mesodermic area is decidedly thickened, and 
consists entirely of the remarkable cells containing a coiled fibre. Along the 
whole extent of this thickening, the inner and outer layers of cells are con- 
tinuous; elsewhere, except along the median line, they are sharply separated. 
Sections show clearly that the inner layer cells along the entire length of the 
thickening are receiving extensive additions from an inward proliferation of the 
overlying ectoderm. The posterior margin of the mesodermic area is drawn out 
into two backwardly directed lobes, which frequently show traces of segmentation 
comparable with that seen in the early stages of the older somites (Fig. 1). 
Fic. 6, X 41. Surface view of entire ovum about 21 days old. Picro-nitric 
acid, borax carmine, balsam. The stained embryo is shown as a transparent 
object. 
The operculum and first pair of gills are well developed, but the chelaria are 
as yet barely visible. The cephalic lobes have become specialized into the semi- 
circular lobes, s./., and the optic ganglia of the lateral eyes, o.g.; the remaining 
portions constitute the brain proper, 47. The ectodermic thickening to form the 
corneagen of the median eyes is shown at m.e. The openings of the tubular 
invaginations of the median eyes and their nerves are shown at /.e.¢. The fused 
distal ends of these tubes will form the median eye vesicle ; the remaining distal 
third of the tubes is converted directly into the corresponding portion of the 
median eye nerve. The proximal portion of the nerves is formed by the separa- 
tion of the nerve fibres from the peripheral surface of the tubes, leaving a collapsed 
epithelial tube behind, which persists for a long time after hatching as a function- 
less remnant. The separation of nerve fibres from the proximal ends of the eye 
tubes takes place before these parts of the tubes fuse with each other, so that the 
