122 PATTEN. [Vou. XII. 
EXPLANATION OF PLATE VII. 
FIG. 50, X 60. This embryo evidently represents an extreme case of median 
fusion, but differs from those on the preceding plate in that there are no indica- 
tions of that V-shaped arrangement of paired organs usually seen when a progressive 
antero-posterior degeneration has followed the median fusion. The cephalic 
lobes, oesophagus, and nervous system are entirely absent. Nothing remains of 
the body but an oblong elevation of thickened ectoderm with three median 
appendages arranged along its summit. Behind the third median appendage was 
a partly invaginated plug of cells, ¢.¢., that may be the remnant of the telopore. 
Besides this there was nothing to indicate to what segment these fused appendages 
belonged, or which was the anterior and which the posterior end of the body. The 
mesodermic area was slightly raised and was pentagonal in outline, with a 
thickened rim. 
Fic. 51, X 60. A large embryo in stage C showing a distinct transverse con- 
striction between the third and fourth thoracic segments. 
The anterior part of the embryo is normal, except in the absence of the cheli- 
cerae. ; 
The posterior portion is infolded between the appendages of the sixth thoracic 
segment to form a deep, circular cavity. The sixth pair of legs, identified by the 
flabella on their outer margin, have been carried inwards by the infolding, till 
they project toward each other from its lateral walls. 
FIG. 52, X 60, sectioned. An hour-glass embryo in stage C. 
The cephalic lobes are deeply depressed, and nearly covered by two lateral 
folds. The chelicerae are absent, the next two appendages are brought closely 
together, and the third pair completely fused to form a thick median appendage, 
ap, extending backwards. 
These three appendages project from a circular depression, bounded on all 
sides by a thickened margin, which anteriorly forms the folds overlapping the 
cephalic lobes. 
The fourth pair of appendages are also fused, af‘, and the fifth are either 
absent, or fused and invaginated to form the pit just back of the base of the 
preceding pair, ap°. 
The sixth pair are small and devoid of flabella, but are nearly normal in shape 
and position. 
The posterior part of the thorax and a part of the abdomen are nearly sur- 
rounded by a marginal fold, well developed posteriorly, but not extending across 
the median line in front of the fused appendages of the fourth segment. 
FIG. 53, X 60, sectioned. A remarkable embryo in stage C that has under- 
gone extensive reduction and fusion. As nearly as one can determine by a study 
of surface views and cross-sections, the following changes have taken place. The 
cephalic lobes, oesophagus, and chelicerae have disappeared, leaving hardly any 
recognizable traces behind. The second thoracic appendage on the left is nearly 
normal, that on the right, a low, irregular papilla. Between the two, in sections, 
traces of a double nerve-cord may be seen. Back of this point, the nerve-cords 
fuse and finally disappear as such, near the large, irregular, median protuberance 
that probably represents the fused appendages of the third segment, ap >. 
