VARIATIONS IN LIMULUS POLYPHEMUS. iy 
EXPLANATION OF PLATE VI. 
Fics. 39-49 illustrate the more important phases in the formation of the 
inverted V-shaped embryos. These embryos are formed by the fusion in the 
median line of the corresponding right and left organs of each metamere. The 
organs nearest the median line are the first to unite, forming in that way an 
unpaired organ, having the characteristic features of each member of the pair. 
The unpaired organ thus formed then decreases in size, and finally disappears. In 
its place the organs next to it, on the same metamere, unite, and in turn degener- 
ate; and so on till the whole metamere has disappeared. The process seems to 
begin in every case in the most anterior metameres ; and in the most typical cases, 
as soon as the first unpaired organ, formed in, say, the first thoracic metamere, 
has disappeared, the same organ is found unpaired in the second metamere; and 
by the time that has disappeared the unpaired condition of that organ obtains in 
the next following metamere, and so on, till every paired organ has become median 
and unpaired, and then disappeared. In the last phase of the process, if realized 
in full, there would be nothing left of the embryo but a single unpaired organ, 
situated at what was the posterior end of the body, and formed by the median 
fusion of the most laterally situated paired organ of the last metamere. 
Such a condition has not been observed, the nearest approach to it being an 
embryo of which nothing remained but the mesodermic area and a posterior 
unpaired process, representing either the last thoracic appendage or the tail lobe. 
In very rare cases one of the posterior pair of appendages may fuse in the 
median line, while there is no indication of fusion in front of that point. But in 
such cases there is no evidence of a progressive median fusion and degeneration 
extending toward the anterior end. 
There is another exception to the median fusion and progressive antero- 
posterior degeneration seen in the hour-glass embryos shown in Pl. IV. 
FIG. 39, X 60, not sectioned. This embryo is instructive, as it is apparently 
in the early stages of median fusion. The cephalic lobes are reduced to a thin, 
circular disc, showing no trace of separate optic ganglia and cerebral hemispheres. 
The oesophagus is a shallow pit in the centre of the disc, and the cheliceral 
segment has disappeared entirely. The appendages of the fifth and sixth thoracic 
segments are nearly normal in position, but in passing forward from this point the 
appendages decrease in size and approach more and more the median line, till in 
the second segment they have nearly united with each other. The nerve-cords 
terminate back of either the third or the fourth pair of appendages, but as this 
embryo was not sectioned that could not be determined with certainty. 
The end of the left third appendage is invaginated. The abdomen is normal 
except for the rather prominent anal plate. 
FIG. 40, x 60. This isa very unusual form, and the only one of its kind observed. 
Median fusion has taken place at both ends. The dorsal organs are very con- 
spicuous, and as they always lie opposite the fourth pair of appendages we can see 
that the following changes have taken place : 
The cephalic lobes, oesophagus, and first thoracic neuromere have disappeared. 
The appendages of the second pair have fused completely, and those of the third 
pair have approached each other, preparatory to the same change. The fourth 
pair is nearly normal. ‘The fifth appendage on the right is invaginated for half its 
