No. 1.] VARIATIONS IN LIMULUS POLYPHEMUS. 77 
if the embryo is well developed before division takes place, 
the head ends will meet each other on the side of the egg 
opposite to the tails and will thus tend to prevent their further 
separation. 
In Fig. 93 separation of the two embryos has taken place, 
as shown by the arrows, in a manner similar to that in Fig. g2. 
But before the new sixth thoracic appendage and those of the 
abdomen were produced, median fusion and antero-posterior 
degeneration took place in the left-hand embryo in the manner 
so frequently observed in single embryos. See Pl. V. The 
cephalic lobes have disappeared, and the first three thoracic 
metameres have fused in the median line, leaving nothing but 
a minute pit to represent the second pair of appendages, and 
a small, median papilla to represent the third and fourth. 
In Fig. 94 the same process is carried further. In the way 
the egg now stands, the common axis of the two embryos was 
originally nearly vertical, as in Fig. 95. The lower one then 
moved upwards past the left side of the other one to its present 
position. It now occupies the free surface of the yolk between 
the dorsal margins of the right-hand embryo. Median fusion 
and antero-posterior degeneration then followed, whether before 
or after separation cannot be determined, reducing the left- 
hand embryo to the posterior part of the thorax and the 
abdomen. The latter has a very conspicuous tail lobe similar 
to that in Pl. V, Fig. 48. 
It is seen on comparing Figs. 90, 92, and 98, that one of the 
hearts and tail lobes must have belonged to the original em- 
bryo; the others must be entirely new. For example, in Fig. 92 
the heart and tail lobe just above y will be formed by the con- 
crescence of the right and left margins of the old embryo, 
while the heart appearing at x will be entirely new. The two 
sides are so much alike in Fig. 97 that we cannot tell which is 
the old half and which the new. Now if the tail ends of 
these embryos should grow past each other, as in Figs. 94 and 
96, then one embryo would carry off, according as it passed to 
the right or left of the other, either a new tail and a new heart, 
or the old ones. If we knew whether the heart of the original 
embryo in Fig. 97 was to the right or the left, as the figure now 
