No. 1.] VARIATIONS IN LIMULUS POLYPHEMUS. 123 
Following the fused appendages is a rather broad expanse, covered by a thin 
(single?) layer of cells. 
The next thickening, af‘, is elongated transversely, and probably represents 
the fused appendages of the fourth segment. 
The next two appendages of the left side are of full size and normal, but owing 
to the absence of the corresponding opposite appendages and nerve-cord they 
have been twisted spirally toward the right. 
The abdomen is absent. At what represents the posterior end of the embryo 
is a deep, tubular invagination, 4/., with thick walls, from which have arisen 
innumerable cells that form a dense cloud in the surrounding yolk. The embryo 
is therefore to be regarded as an hour-glass embryo, still further modified by the 
absence of the right posterior half of the thorax. It should be compared with 
the following one. 
The surface of the embryo is covered with bacteria. 
FIG. 54, X 60, sectioned. This remarkable embryo in stage D-Z belongs to 
the hour-glass type, but it is modified by the median fusion of the anterior end, 
and by the absence of the right half of the posterior end of the thorax. It seems 
to be the result of still further progress along the lines followed by the embryo 
shown in Fig, 53. 
In such cases as this it is very difficult to determine just what changes have 
taken place, especially as regards the amount of nerve tissue, if any, that is left, 
and the segments to which the remaining appendages belong. 
As near as can be determined, the changes have been as follows : The cephalic 
lobes are absent, and in their place is a broad, triangular, ectodermic plate, 
beneath which is an unusually large number of yolk cells. 
The marginal fold has contracted anteriorly to form a thick rim around an 
oval depression. At the anterior end of this depression is seen in surface views a 
dark spot, which is the optical section of a very long oesophagus-like tube extend- 
ing vertically into the yolk, about as far as the adjacent appendage is long. The 
tube is not closed at its inner end, and its walls are folded longitudinally, as in a 
true oesophagus. 
Back of this tube is a long irregular appendage arising at first directly upward 
from the bottom of the depression, and then bending over to the side, so as to 
lie flat on the surface of the egg. Back of this is another appendage; it is bent 
double, and in such a way that its distal end points toward the median line, 
toward a point a little in advance of its proximal end. 
The lateral growth of these two appendages at first seemed to me to indicate 
that the whole right half of this portion of the embryo had disappeared, and that 
the left half had rotated on its own longitudinal axis 180° toward the right. But 
after a careful examination of the sections, I failed to find any traces of either 
nerve-cord ; and, on the whole, the sections seemed to indicate that each appen- 
dage had been formed by the fusion of a pair, and, as is so frequently the case in 
such embryos, the nerve-cord had entirely disappeared. 
The posterior end of the embryo, which is separated from the anterior one by 
thin, characterless layers of ectoderm and mesoderm, consists of a deep, thick- 
walled and irregular sac, from the posterior wall of which arise two long-pointed 
appendages, extending upward and forward. There are some parts of the wall of 
the sac that look histologically like parts of a nerve-cord, but the whole layer is 
so folded and crowded together that nothing in regard to this point could be 
