68 PATIL. [VoL. XII. 
sense-organ, d@.0., is the first thing to suggest a break in the 
apparently homogeneous series of thoracic metameres. 
But we frequently see in embryos which in most respects do 
not depart from the normal type, quite constant differences in 
the arrangement of the thoracic appendages, which may be 
regarded as indications of the cleavage plane in question. In 
Pl. III, Fig. 23, the potential fissure plane 1s indicated by an 
increased distance separating the third and fourth pairs of 
appendages. This would be of little importance perhaps taken 
alone, but such cases are frequently seen, so that it probably 
has the significance attached to it. 
Another class of cases illustrating the same thing consists 
of embryos in which the ¢hree anterior pairs of appendages have 
a markedly different direction of growth from the posterior 
ones, suggesting the characteristic difference in appearance 
between the mandibles and maxillae of an insect embryo and 
the thoracic appendages. These cases are also comparatively 
common, typical cases being shown in PI. III, Figs. 19 and 22. 
None of these cases would attract attention if it were not for 
the fact that the constriction about to be described occurs 
between these two sets of thoracic metameres. 
A most beautiful example of transverse fission, and one that 
throws a good deal of light on the ones about to be considered, 
is shown in Pl. IV, Fig. 29. This case is, however, compli- 
cated by the partial degeneration of the left half of the thorax, 
and the complete absence of the left half of the abdomen. 
The constriction is due to the presence of a transverse line of 
degeneration having its greatest intensity along the fourth 
segment. 
In Pl. VI, Fig. 51, is an obvious constriction between the 
third and fourth thoracic metameres. When the constriction 
is more conspicuous, it has evidently been preceded by fusion 
of the right and left halves, the degree of fusion diminishing 
from the point of greatest constriction toward the anterior and 
the posterior end. This is beautifully shown in the lower 
embryo of Pl. VIII, Fig. 98. As every pair of appendages is 
present in this case, there is no question about the exact point 
of constriction. 
