142 PATTEN. 
FIG. 97, X 33. This embryo is in an older stage than the preceding one. It 
is an almost perfectly symmetrical double embryo, tail to tail as in Fig. 92. 
Median fusion and antero-posterior degeneration have affected both embryos, 
but the lower one more than the upper. In the latter, everything as far as the 
fourth segment has disappeared. The dorsal organs are very large, and are 
supplied on one margin with considerable black pigment, a condition that has not 
been seen in the normal embryos. They have approached the median line, but 
have not fused with each other. The appendages of that segment, ze. fourth, 
have fused, and the single appendage thus produced is reduced to a small conical 
projection, af4. The fifth pair have also fused, forming a long zigzag appendage. 
The sixth pair have fused at the base to form a large oval vesicle, from the 
summit of which project the separate ends of the reduced appendages. The 
chelaria and abdominal appendages are normal. 
In the lower embryo, 2, the ‘dorsal organs” on the fourth segment have 
fused. Everything anterior to that has disappeared, but the appendages of the 
fifth and sixth segments remain as elongated, crumpled, unpaired organs. 
The nerve-cords extend without interruption or modification from one animal 
to the other. There are two perfectly normal hearts and two abdominal lobes, 
both organs being shared in common by the two embryos. 
Fic. 98, X 33. This is a very interesting case, as it shows clearly three entirely 
separate phenomena, z.e. (1) longitudinal division; (2) median concrescence; 
(3) transverse fission. The two latter conditions have been seen in the single 
embryos previously described. The two embryos now form almost a straight 
line. The abdomen of the original embryo is intact, and would have become, 
in all probability, the abdomen of embryo A. A remarkable fact is the obvious 
“weakness” of the new half of embryo 4, as compared with its old, right 
side. This is shown by the absence of its second and fourth appendages, and 
the fourth neuromere. In place of the fourth appendage is a minute pore that 
may be the invaginated remnant of the same. Embryo Z& is a beautiful example 
of the hour-glass type, the constriction occurring in the usual place, between the 
third and fourth segments. The appendages of the third and fourth pairs are 
fused in the median line. In front of the third pair, and back of the fourth, is 
a gradual diminution of the concrescence. The large size of the chelicerae 
in embryo Z is surprising, considering the otherwise reduced condition of the 
embryo. 
FIG. 99, X 35. This is a very rare form, and the only one of the kind I have 
seen. It at first sight seems to belong to a different class from the preceding, 
and to have been produced in a different manner. However, it is easily explained 
by assuming that during fission, like that in Fig. 91, median fusion and antero- 
posterior degeneration destroyed the anterior part of embryo Z& as fast as it was 
formed. Two new nerve-cords extended to the tip of the abdomen, and a row of 
unpaired appendages, extending from the fourth abdominal to the fifth thoracic 
segment, have been formed. 
