No.1.] VARIATIONS IN LIMULUS POLYPHEMUS. I4I 
FIG. 93, X 33. The process seen in the preceding figure has evidently begun 
here. But before the two new halves were completely formed, the left-hand 
embryo began to concresce along its own median line. The cephalic lobes and 
first two segments have disappeared in that way, exactly as in single embryos 
(see Pl. IV). The sixth pair of appendages has not been formed in the new 
halves. 
FIG. 94, X 33. Here the process seen in the preceding figure has been carried 
further. Fission probably occurred at first, as in Fig. 92; degeneration of one 
embryo then followed, as in Fig. 93; and finally the two embryos separated, 
moving tail first in opposite directions to the position they now occupy. The 
right-hand embryo is apparently normal; the left-hand one has been reduced by 
antero-posterior fusion and degeneration, till the last two pairs only of thoracic 
appendages remain. The fourth pair have united in the median line. The 
“dorsal organs,” d.o., are still separate and very distinct. Between them is a 
small mass of cells, the remnant probably of an unpaired appendage. This 
embryo, with its five legs and large tail lobe, is very similar to that in Pl. V, Fig. 48. 
Fic. 95, X 33. Thisis a double embryo which, at one stage of its development, 
was probably like that in Fig. 92. The normal upper embryo still occupies its 
former position tail to tail with the lower one, which median concrescence and 
antero-posterior degeneration has reduced to a small, median appendage, project- 
ing from an oval depression. The abdomen is a narrow oblong thickening, and 
at the opposite extremity of the body is a median depression that may represent 
the fused dorsal organs (compare Fig. 97), or the remnants of an oesophagus. 
On either side of this much-reduced embryo is an irregular, dark band, formed 
by the concrescence of the margins of the mesodermic areas of the two embryos, 
and probably representing two rudimentary hearts. 
Fic. 96, X 33. This is a much older embryo than any of the preceding. It 
has passed successively through approximately the same stages seen in Figs. g2- 
94. Thé exact sequence of events, of course, cannot be determined ; but we can 
understand the position of appendages in the right-hand embryo, by assuming 
that after a tail-to-tail condition was reached median fusion and antero-posterior 
degeneration of the lower embryo followed, somewhat as in Fig. 93. Then the 
tail of the lower embryo pushed past the tail of the upper, going to the right of it 
instead of to the left, as in Fig. 94. But meantime the margin of the thorax of 
the larger embryo had grown so large that it had practically preémpted the 
territory in that region ; the tail of the smaller embryo was thus forced to bend 
to the right, through an angle of about 90°. The curvature thus produced of the 
axis of the smaller embryo is shown by the arrow 2, and by the dotted line which 
runs along the median line from the fourth thoracic appendage, af 4, to the abdo- 
men. The latter is perfectly normal and well developed. The cephalic lobes and 
first three neuromeres have disappeared, through median fusion and antero- 
posterior degeneration. The appendages of the fourth and fifth segments have 
fused along the curved median line, and are very much reduced in size. The 
sixth pair, af, are still quite large, and fused only at their base. The flabella, 
ji., are very large, and each is separated by a relatively wide space from the base 
of its respective appendage. 
Just under the end of the left sixth appendage is a small projection that 
cannot be accounted for, as its position is such that it cannot be brought into 
line with any of the other appendages. It may be the left flabellum of embryo 4. 
