No: tr.) VARIATIONS IN LIMULUS POLYPHEMUS. ree 
In conclusion, therefore, tt may be stated that transverse fission 
occurs in the region of the fourth thoracic segment, dividing the 
body into two parts, which show different morphological characters 
and different degrees of vitality. The separation of the two parts 
7s the result of a constriction brought about by the successive 
median fusion and degeneration of the organs lying along that 
segment, the most median, and therefore the oldest and most 
specialized, fusing and degenerating first, and the others follow- 
ing in the order of their position on the segment. There is a 
gradual diminution of the effects of degeneration both in front 
of and behind this line. A similar line of degeneration ts seen 
zn the region of the cheliceral segment, just back of the cephalic 
lobes, and in the region of the first abdominal or cheliceral seg- 
ment, Fig.6. Further evidence of degeneration at these points is 
seen in normal adult animals, in the small size of the organs on 
these segments, and in their tendency to unite in the median 
line. In the abnormal embryos it is shown by an exaggeration 
of these conditions, resulting very frequently in the absence of 
entire segments. 
The inherent tendency to diminution of growth along such lines 
7s what has, in all probability, led to the division of the body of 
arthropods into successive regions, such as cephalic lobes, pro- 
thorax, thorax, abdomen, post-abdomen, etc. 
B. MEDIAN LONGITUDINAL FISSION; DOUBLE AND TRIPLE EMBRYOS. 
This method of forming multiple embryos is comparatively 
common. It often begins at a late period, after the full number 
of normal appendages and metameres is formed. 
The first steps in the process have not been observed, but 
a study of Fig. 90, where fission has not progressed very far, 
shows what the initiatory processes must have been like. 
There can be no doubt that we have actual fission here, and 
not fusion of two originally independent embryos. It is also 
clearly shown by such cases as that in Fig. 98, where the left 
half of the larger embryo is defective, owing to the lack of the 
formative material necessary to produce an entire new half; 
and especially by the fact that in all these cases the embryos 
