80 PALE. [Vou. XII. 
the second, and the small size of the invaginated fourth, 
appendage. There is no indication of segmentation lateral to 
these appendages, and the left half of the neuromere opposite 
the fourth appendage is apparently absent. The weakness of 
this half is further shown by a gentle curvature of the head 
toward the left, although at this stage in other double embryos 
it is curved to the right. 
In Fig. 97 is a very late stage (about the trilobite stage) of a 
double embryo. The two embryos are tail to tail in a straight 
line, and so symmetrically developed that there is no indication 
whatever of the direction in which the two embryos separated 
from the primitive median plane. Median fusion and antero- 
posterior degeneration of each embryo has progressed as far as 
the fourth thoracic segment. As indicated by the large seg- 
mental sense-organ and the appendages, fusion and degenera- 
tion have progressed farther in the lower embryo than in the 
upper one. Interesting features of these two embryos are the 
two perfect hearts and tails, extending laterally at right angles 
to the continuous nerve-cords, and the large flabellae, which 
look like a separate set of appendages. 
In Fig. 96 is what appears to be a case of division of a dif- 
ferent nature from those just described ; but a careful examina- 
tion will show, I think, that it is after all the same. We can 
easily and satisfactorily explain its condition by assuming that 
longitudinal division gave rise to two embryos in a straight line 
tail to tail, and that they separated and pushed past each other 
in opposite directions. The anterior end of the right hand 
embryo is turned vertically downward and has undergone me- 
dian fusion and degeneration. This resulted in the fusion of 
the fourth and fifth pairs of thoracic appendages, leaving the 
sixth pair and the abdomen in a nearly normal condition. The 
abdomen, as indicated by the dotted line, has finally been thrown 
sharply to the right by the growth of the left side of the 
thorax of the larger embryo. The result is an embryo much 
like that in Fig. 97, only its longitudinal axis is bent at 
right angles, the anterior portion of what is left being in its 
primitive position, parallel with the axis of the larger 
embryo. 
