Nov T-] VARIATIONS IN LIMULUS POLYPHEMUS. 5 
around the stomodaeum represents the remnants of the cephalic 
lobes. 
What appears to be a still more degenerate condition is 
shown in Pl. VI, Fig. 69. Here the inner layer cells of the 
margin of the germinal area are breaking up into dense, irregu- 
lar masses of degenerating cells, while every trace of the head 
is absent. 
Degeneration of fused organs does not always proceed in 
that way, as is illustrated by the remarkable embryo shown in 
Pl. VI, Fig. 50. This is the only instance observed in which 
three successive metameres show the same degree of fusion. 
There is no indication as to what appendages are represented 
in this embryo, probably the last three thoracic, as we have 
seen that they have greater vitality than any other organs of 
the body. 
Similar conditions are shown in Figs. 52-58, but complicated 
by partial transverse fission. See Section VI. 
The only exceptions observed to the law of fusion and degen- 
eration illustrated by the preceding figures—to which many 
more might have been added — are shown in PI. V, Figs. 40 and 
45. In the first, the cephalic lobes and cheliceral segments 
have disappeared. The second and sixth pairs of appendages 
are fused, so that the whole embryo is spindle-shaped. The 
identity of the appendages is determined by the ‘dorsal 
organs,” which are very plainly developed. This embryo, then, 
seems to furnish a case of median fusion and degeneration at 
both ends. In Fig. 45 the last pair of legs has fused. It 
probably represents the fifth pair, the sixth having dis- 
appeared. 
Nearly all of the older multiple embryos show in one or 
more of the component individuals the typical antero-posterior 
fusion and degeneration. But a modification of this process 
occurs there of extreme interest. In such cases the fusion is 
greatest between the third and fourth thoracic metameres, and 
diminishes from that point both forward and backward, pro- 
ducing what I have called hourglass embryos. The con- 
striction thus produced may separate the anterior and posterior 
regions completely. This condition is very beautifully shown 
