148 PATEEN, 
EXPLANATION OF PLATE XI. 
The number opposite each figure corresponds to the number of the embryo 
that was sectioned. The exponent indicates the position and number of the sec- 
tion, and agrees with the number opposite the dotted section line, s, in the surface 
views. 
The sections are not intended to show histological details, but merely as a 
help in the interpretation of the surface views. 
Fic. 81,2, X 200. Two longitudinal sections of the brain, optic ganglion, and 
invaginated appendage. 
Fic. 9°, 5,7. Cross-sections of the cephalic lobes, to show the nature of the 
fold that grows over the brain. 
Fic. 101,2,8. Longitudinal sections showing the ganglionic fold over the 
optic ganglion, and the brain pits. 
Fic. 10 4, X 200. Longitudinal, vertical section of an invaginated appendage. 
Same section as 101, but shows a part farther back, and more deeply invaginated. 
Fic. 111, X 400. Section of a marginal vesicle, showing fatty degeneration of 
the mesoderm cells. 
Fic. 201, 2,3,4, X roo. Cross-sections, showing absence of nervous system at 
the anterior end, and the medianly fused appendages. 
Fic. 68 1,2, X 100. Cross-sections, showing invaginated, marginal thickening. 
A. Same more highly magnified, to show the bacteria-like dots on the surface. 
Fic. 731,2, X 100. Section of a late stage of a degenerate embryo, showing 
absence of organs and histological specialization. 
Fic. 771,2, X 100. Section of embryo similar to that in Fig. 66. It shows a 
remarkably large, horseshoe-shaped body cavity, possibly formed by the fusion of 
several somites. The cells lining the floor of the cavity are flattened and mingled 
with fibres, so it has the character of connective tissue seen in very old stages. 
The somatic mesoderm is on the contrary composed of lymphoid cells, which are 
here and there wedged in between the inner ends of the columnar, ectodermic 
ones, as though they arose from the latter by inward proliferation. 
On each side of the cavity is a thickened cord of mesoderm cells, which seems 
to represent the thickened rim of the mesodermic area. Such a relatively 
enormous body cavity (?) has not been seen in any other embryos. 
Fic. 831,2,8, X 100. Cross-sections of an invaginated embryo, showing ab- 
sence of recognizable nerve-cords and appendages. 
Fic. 84, X 100. Cross-section of an invaginated embryo, showing the last 
traces of the nerve-cords, appendages, and eyes. 
Fic. 85 1,2, X too. Cross-sections of invaginated embryo, showing the relation 
of the foldings. 
Fic. 89. Section of a very old embryo, reduced to a formless mass of 
cells. Of the latter, some are multiplying by karyokinesis, others degenerating ; 
some are lymphoid, others finally are the very remarkable amoeboid cells, con- 
taining coiled fibres. These cells are found in great numbers in the marginal 
thickenings of the mesodermic area of very old embryos, and are also found in 
great numbers in the adult. &. Two fibre cells enlarged, showing the fibre 
lengthwise and in optical cross-section. 
