130 IBS GIVE [VoL x1 
EXPLANATION OF PLATE VIII. 
Fic. 70, X 60, not sectioned. A very much reduced embryo probably in 
stage C. There are faint indications of the nerve-cord and cephalic lobes and 
oesophagus. Two appendages are present on the right; on the left the corre- 
sponding appendages are invaginated. 
Back of the embryo is a long band of inner layer cells formed by the concresced 
margin of the mesodermic area. The length of this band affords some indication 
of the advanced stage of development of the embryo, and the amount of reduction 
it has undergone. 
The mesodermic area is nearly circular and very large, only the posterior por- 
tion being shown in the figure. Its extent may be approximately determined by 
continuing the curved lines, c.m.a. 
Fic. 71, X 60, sectioned. This embryo is so flattened and distorted that its 
parts are hard to identify. Beginning at the anterior end of the embryo and 
passing backward, we find first a deep depression, at the bottom of which is a 
well-defined nerve-cord. From the right wall of the depression arises a large 
appendage, af", that projects over and conceals the nerve-cord. Its mate on the 
opposite side is absent. 
Back of the depression, the surface of the embryo rises to nearly the level of 
the ovum, and here is a nearly normal neuromere, 7.#., with an invaginated 
appendage on either side of it, a*. The ectoderm then thins out considerably, 
but thickens again to form an indistinct nerve-cord, .m *, with a large invaginated 
appendage on the left, af °. Following this are two swellings that may represent 
appendages ; and finally this decidedly mixed-up collection of organs terminates 
in a thickened ectodermic disc, which appears to represent the anal plate. In 
both sections and surface views indications of the thickened mesodermic margin 
are seen on the side. Posteriorly the margin is very broad, and, owing to 
concrescence, deeply notched. 
This embryo is in the last stages of degeneration that show recognizable organs. 
In the following figures, even these rudiments disappear, and nothing remains 
but simple layers of mesoderm and ectoderm, complicated by more or less irregular 
foldings. 
In the following figures, 72-78, so far as one may judge from what little 
remains of the embryos, there has been median fusion, and antero-posterior 
degeneration. The embryos were probably laid down throughout their whole 
extent, and then underwent a gradual median fusion and degeneration, coz/ized 
principally, however, to the axial organs, and to the region midway between the two 
extremities. In none of this series is there any recognizable trace of nerve-cord, 
appendages, or other axial organs; although the peripheral mesoderm retains in 
many cases very nearly its normal character. It is assumed in each case that the 
present condition was preceded by one having nerve-cords, appendages, cephalic 
lobes, and sense-organs, and that these have gradually faded away, leaving only 
the mesodermic area in a recognizable condition. Within this area are two (sel- 
dom but one) great masses of mesodermic (?) or yolk (?) cells, either with or 
without an overlying ectodermic invagination. They represent the remnants of 
the head and tail. 

