106 PATTEN. EVOL, xaT 
Fic. 10, X 60. A large embryo, a little older than stage C-D. All the thoracic 
appendages are large. The third pair are completely invaginated, and project 
their whole length into the yolk. The pockets thus formed open to the exterior 
by two sharply circumscribed transverse slits, 7. 
The apex of the fourth appendage of the left side, a/., is invaginated, and the 
base is expanded and folded as though vertically compressed. A section of the 
third pair is shown in Pl. XI, Fig. 10%. The ectoderm is sharply defined except at 
the tip, x, where the arrangement of the nuclei indicates a proliferation of ectoderm 
cells into the yolk. A similar condition has been observed in other embryos. 
The cephalic lobes are modified in appearance by the extensive overgrowth of an 
amnion-like, ganglionic fold along their anterior margin. Longitudinal sections in 
the planes indicated by the lines 1, 2, and 3 are shown in Pl. XI, Figs. 1o!, 10%, 
and 10%. Abnormal depressions in the region of the future cerebral hemispheres 
are seen at 4y.zv., in Fig. 10, and in sections in Fig. 107. 
Near the median line the brain is covered with a layer of ectoderm cells, Fig. 
103, not developed in the lateral areas, Figs. 107 and 107. 
The lateral eyes have moved back to a point opposite the second pair of 
appendages. The large thoracic sense-organs are very feebly developed, if not 
entirely absent. 
There is a conspicuous mass of cells lying in the yolk back of the tail lobe, 
pac. It is formed by the concrescence of the thickened rim of the mesodermic 
area. 
Fic. 11, X 60. This embryo is similar in general appearance to that shown in 
Fig. 10. The thoracic appendages are, however, longer, and there are four pairs 
of abdominal appendages present. The distal half of the third thoracic appendage 
on the left side is invaginated, ¢4.af. The fourth and fifth appendages are 
directed forwards instead of backwards. The brain-pits, 47.2v., are here smaller 
than in Fig. to, and pushed farther forward beneath the ganglionic fold. The 
whole brain and optic ganglia, of.g., are depressed below the level of the ventral 
cord and the margin of the ganglionic fold. The ganglionic invagination, 2.2v., is 
about the same as in Fig. 10. The light area, which indicates the mouth of this 
slit-like depression, gradually runs into the rounded brain-pits on the median side, 
brav. The ganglionic fold is continued as a thickened band, without infolding, 
across the median anterior border of the cephalic lobes. 
The lateral eyes are unusually conspicuous, and lie very nearly on a line with 
the chelicerae. 
A large peripheral vesicle, #v.ve., extends inwards almost to the centre of the 
egg. It is a conical cavity in the yolk, lined with a thick layer of cells, derived 
from an abnormal growth of the thickened margin of the mesodermic area. A 
well-marked depression is present in the abdominal plate that in surface views 
appears to mark the beginning of the proctodaeal invagination. 
FIG. 12, X 60, stage C.D. 
The thoracic appendages have the character of those seen in the early 
phases of stage D, while the abdomen has the characters seen in stage C. The 
apex of the third left leg is reduced in size and deeply invaginated. 
The cephalic lobes, which have run together to form a _ broad, unpaired 
thickening, are thrown to the left by a remarkable growth of the right half of the 
cheliceral segment. The original right chelicera has apparently divided, produc- 
ing appendages a3-4 and a’*. A second division followed, producing a3 and a4, 
