140 PATTIEN, . [Vorcxt: 
EXPLANATION OF PLATE IX. 
In this plate, double monsters are shown in various stages of formation. In 
all the cases I have seen in Limulus, double embryos are formed by fission, or 
more correctly by the gradual intercallation, beginning at the anterior end, of two 
new halves between the old. If there are five paired organs on each segment, 
and a is the most median one, and e the most lateral, then a will be the first 
new organ to appear, and it will appear in the median line of the first segment, as 
an unpaired organ, having the same appearance as each member of the paired 
organ. It divides, and in its place in the same segment will be found an unpaired 
organ like organ 4. But at the same time a new, unpaired organ, like a, will be 
formed in the median line of segment number two. At the next division, organ 
a will be produced in the median line of the third segment, 4 in the second, and 
c in the first; organs a and 6 being now completely formed in pairs in the first 
segment, and organs 4 in the second. This process goes on till two complete 
new halves are wedged in between the old, and two new individuals are produced, 
each individual consisting of an old and a new half. The old halves were 
produced by normal apical growth from behind forward ; the new half was also 
produced by apical growth, but from before backwards. Each new half is a 
mirror image of the other, and at the same time the mirror image of the old half 
next to it; but the new halves are united by their lateral margins instead of the 
median ones. 
The very first steps in this process have not been seen. Hence the manner in 
which the new halves of the cephalic lobes and the oesophagus are produced, can 
only be inferred from the manner in which new organs are formed in the post-oral 
segments. The process of forming two new organs by the division of the 
unpaired ones, takes place in exactly the reverse order that unpaired organs are 
formed by the fusion of paired ones. 
FIG. 90, X 33. The entering wedge formed by the newly produced organs has 
reached the fifth thoracic segment. At the end of the wedge is an unpaired 
neuromere ; in the next in front of it are two new halves of a neuromere and an 
unpaired appendage. In the next segment in front of that are two complete and 
newly formed chelicerae. 
FIG. 91, X 33. Here the process has progressed still further. The division of 
the new appendage in the third segment is almost completed, the separation 
extending from the tip nearly to the base. In the fourth segment the new 
appendage is perfect, but shows no trace of division. 
FIG. 92, X 30. The growth of the new halves is practically completed, forming 
two distinct embryos, which have, however, an abdomen in common. The new 
halves lie on the upper side, as the egg is placed in the figure, the old ones being 
below. Each of the old halves has been rotated on its tail end 90°, one to the 
right, the other to the left. They have been forced apart in this way, at first by 
the wedge-like ingrowth of the new nerve-cords. But as the organs lateral to the 
two nerve-cords develop, they push the previously formed parts still farther right 
and left. This goes on till the new embryos form a straight line tail to tail. 
Further rotation of the old halves will be stopped by the interference of the 
lower margins of the mesodermic areas at y. 

