126 PATTEN. [Vou. XII. 
doubt, the brain and optic ganglia. The deviations from the normal, then, 
in this embryo are as follows: The whole right optic ganglion and a part 
of the right cerebral hemisphere are absent. On the left side the cephalic 
lobe is perfect, and one can distinguish in sections the minute pit that develops 
into the lateral eye, Ze. The chelicerae are probably absent, the two pairs 
of appendages now present belonging to the second and third thoracic 
segments. The appendages of the fourth segment have fused in the median 
line, and the marginal fold has closed in behind them. From this point in 
the marginal fold a faint cloud of yolk cells extends backward a consider- 
able distance to a small thick-walled depression in the ectoderm, 4. From the 
posterior wall of the shallow depression arises a minute papilla, which may be the 
remnant of the tail lobe. (Compare Figs. 27,43, and 64.) It seems too far back to 
represent the fused appendages of the last thoracic segment. The anterior part 
of the depression is still further invaginated to form a short, conical pocket, 
directed diagonally forward into the yolk. The inner end of the pocket is con- 
tinuous with a great mass of cells which seem to be wandering into the yolk 
there to degenerate and disappear. It would appear, therefore, that transverse 
fission has taken place, and that contrary to every other case that has come under 
my notice, except, perhaps, Fig. 45, the subsequent fusion and degeneration have 
been greatest in the posterior half of the thorax. 
FIG. 61, X 60, sectioned. The mesoderm and ectoderm are much thickened 
and concentrated along the axial portion of the embryo. There is no distinction 
into head, thorax, and abdomen. The body of the embryo forms a deep, elon- 
gated furrow, constricted laterally to form a chain of four or five clearly marked 
dilatations, each one of which represents a metamere. At the bottom of two 
of the dilatations are shallow, slit-like infoldings that represent the remnants of 
invaginated thoracic appendages, za. In sections there is no recognizable remnant 
of the nerve-cords to be seen. 
The mesodermic area is sharply defined on its periphery, where there is the 
usual thickened rim, but it contains very few mesoderm cells, except beneath the 
axial portion, where they form a very thick, compact layer ten to fifteen cells deep. 
It is this deposit of cells which forms the dark ring seen in surface views. 
This embryo is also remarkable for the fact that the surface ectoderm of the 
entire mesodermic area is covered with a very thick deposit of a soft, vitreous 
exudation, resembling chiten. Over the axial furrowit has accumulated in great 
botroidal masses, among which in each section may be seen four or five rounded, 
amoeboid cells that look like those seen in the peripheral vesicles. There is no 
indication as to where these isolated cells, which are separated by considerable 
distance from the embryo, come from, or what their function may be. Their 
occurrence at this place suggests a rudimentary amnion and serosa, but otherwise 
there is nothing to indicate that they develop in the manner characteristic of these 
organs. 
FIG. 62, X 60, not sectioned. Embryo with three pairs of appendages, per- 
haps the fourth to sixth pairs. The cephalic lobes are deeply depressed, and 
separated from the thorax by a sharp, transverse fold. The rim of the mesodermic 
area is very distinct, and shows clearly the posterior concrescing margins, and the 
post-anal cloud of yolk cells between, #.a.c. 
The abdomen is deeply depressed, forming a cavity with nearly vertical walls 
surrounding a triangular opening. 
