No. I.] VARIATIONS TN LIMULUS POLYPHEMUS. 8I 
In Fig. 100 is a much older embryo, with the remains of a 
second rudimentary one attached to its right side. Separation 
of the two embryos probably took place in the direction indi- 
cated by the arrows,— the left-hand embryo undergoing 
median fusion and antero-posterior degeneration. The re- 
maining appendages are so twisted and obscure that their 
identity could not be certainly determined. They appear to 
represent the fused third, fourth, and fifth pairs of thoracic 
appendages arranged in single line. The sixth pair are fused 
at the base, leaving the ends free. The first two pairs of abdom- 
inal appendages have also fused, something that rarely occurs 
with them, to form two median, tongue-like projections, each 
of which is bent almost at right angles. 
On examination from the dorsal side, Fig. 1o1, the outlines 
of the two embryos are distinctly seen. The median dark 
streak in the smaller embryo probably represents the rem- 
nants of the oesophagus, or perhaps the heart. The large semi- 
lunar band of cells consists of the fibre cells that I have 
described elsewhere, and represents the concresced margins of 
the mesodermic area. It should lie on the anterior dorsal 
surface of the thorax, but is here thrown forward towards the 
ventral surface. 
The only instance observed in which there seems to be a 
deviation from the method of forming double embryos, just 
described, is shown in Fig. 99. This embryo was accidentally 
destroyed before a finished drawing of it was made, but 
it had been completely outlined and carefully studied. There 
is no question therefore about the correctness of the details of 
structure, as far as they are given. 
At first sight the right-hand embryo appears to have arisen 
by longitudinal regeneration from the left side of the larger 
one. Aside from the fact that no indication of such a process 
has been seen elsewhere, it is difficult to imagine in detail the 
method by which it was brought about. It is not necessary to 
discuss these possibilities, as long as we can reduce this type 
to the usual one by assuming that the form of degeneration 
frequently seen in single embryos, has affected one of the 
embryos almost coincident with its separation from the other. 
