Nosiay | VARTA TIONS IN EIMCLUS: POLYPHEMUTS. 133 
many degenerating nuclei in the ectoderm and mesoderm of the posterior thicken- 
ing, but comparatively few in the anterior one. 
There is a gently elevated, thickened band of ectoderm, connecting the two 
invaginations, but it does not show any histological differentiation, and the 
slightly thickened layer of mesoderm underlying it is not segmented or otherwise 
specialized. 
FIG. 77, X 60, sectioned. In this embryo everything has disappeared except 
two masses of cells, one at either end of what was the body of the embryo. 
There is a small cluster of mesodermic cells at the anterior end, and a similar one 
at the posterior extremity. Over the latter is a thickened ectodermic area with a 
rather deep, vertical invagination in its centre. There is no indication of any 
ectodermic or mesodermic thickening connecting these two masses of cells. 
The outlines of the mesodermic area are not well defined. 
Fic. 78, X 60, sectioned. Embryo with well-defined, circular, mesodermic 
area. There is an axial band of thickened ectoderm, slightly enlarged at either 
end, and beneath it is a broader band of mesoderm, which becomes enlarged at 
either end to form two great irregular masses of cells, c.¢. The latter project 
deeply into the yolk, and from them radiate pseudopodia-like processes, composed 
of dense masses of cells. Over the centre of these masses, ectoderm and meso- 
derm are continuous, as though the inner cells arose by proliferation from the 
outer ones. The outer layers at these two points, however, contain no karyokinetic 
figures, but instead, numerous degenerating nuclei. 
The ectoderm over the posterior mesodermic enlargement is composed of a 
very thick layer of columnar cells, but shows no noteworthy histological differen- 
tiation. 
No bacteria (?) were visible on the surface of the ovum, and the tissues 
appeared healthy and normal. 
Fics. 79-89. In this series, the processes of degeneration have followed 
somewhat different lines than in the one just described (Figs. 72-75). But we do 
not assume that there is any sharp distinction between the two series. 
We have here in some instances undoubted evidence that the embryos in 
question represent mere remnants of highly developed individuals, or to put it 
more accurately, some of these embryos may consist of a single recognizable 
organ only, which may be in the condition seen in normal embryos of stage D, 
while every other organ characteristic of this stage has either failed to develop or 
has completely degenerated. 
This is especially true of the embryo shown in Fig. 82, where the large tail 
lobe and the concrescing mesodermic margins suggest the conditions seen in stage 
A and D (see Figs. 5 and 6), while the remainder of the body of the embryo is 
reduced to a mere sac, invaginated into the yolk. 
In the cases that seem to lead up to this condition, the body of the embryo is 
usually pinched and narrowed and much depressed, with high, conspicuous, 
marginal folds that approximate each other toward the median line, Fig. 70. 
The appendages may or may not be present, but if present, they are usually 
invaginated. 
Practically all conditions, from normal embryos, and ones like those in Figs. 
61, 62, and 67, to those shown in Figs. 81 to 89, have been observed. 
From the first condition towards the last there is a gradual reduction of sense- 
organs, nervous system and appendages, till nothing is left of the embryo, but a 
