134 PATTEN. [VoL. XII. 
more or less clearly defined mesodermic area, with a central thick-walled sac, 
representing the remnant of the axial portion of the body. This sac may be a 
single one, regular in outline, or it may be more or less irregular, as though com- 
posed of two or more sacs arranged one behind the other along the median line 
(Fig. 81). 
These sacs may be of a different nature in each case. In Fig. 61, the dilata- 
tions of the common furrow represent single metameres, from which the ap- 
pendages and nerve-cords have nearly disappeared. In Figs. 72 and 79, each sac 
probably represents the remnants of several segments. These embryos may 
belong to the hour-glass series, where there is a transverse constriction between 
the third and fourth thoracic segments dividing the embryo into two distinct 
parts. Subsequent degeneration of the appendages and other organs would 
reduce each part to a separate sac. 
When the sac is comparatively large, and communicates with the exterior by a 
small opening only, as in Figs. 82, 83, 84, 85, and 86, a study of the sections 
through the sac reveals an unexpected complexity of structure, the walls being 
thickened or variously curved, so as to suggest that the cephalic lobes, with their 
various ganglia, and the anterior part of the nerve-cord, still persisted, while the 
appendages had disappeared. I have not been able to satisfy myself that such 
was really the case, for the various parts are thrown so much out of position, that 
it is impossible to identify them. But the whole histological character of the 
thickened walls of these sacs resembles nerve tissue more than anything else. 
For this reason, and owing also to the shape of the sacs, cross-sections of them 
resemble cross-sections of embryonic vertebrate brains. This is true of Figs. 83 
and 84, especially the latter, where the tubular outgrowths from the anterior part 
of the sac recall the similar outgrowths to form the lateral eyes in vertebrates. 
These sacs themselves degenerate. Their walls collapse and lose their 
character as distinct cell layers, till there is left merely a central mass of cells ina 
much contracted mesodermic area. All distinction between these two parts, z.e. 
ectoderm and mesoderm, is gradually obliterated, and what then remains seems 
to disappear completely by absorption. It would be a very difficult matter to prove 
that this complete absorption really does occur, for one cannot follow any single 
individual through the process, but there is no difficulty in finding among these 
degenerating eggs a few apparently healthy ones in which there is no trace of 
nuclei or cell clusters to be found. There is also no difficulty in finding ova, showing 
various stages, from embryos composed of a barely recognizable cluster of cells, 
to ones like that in Fig. 89. These ova without embryos may be three or four 
months old, and if they had failed to develop in the first place and died, it is 
difficult to understand how they could be preserved so long without putrefaction. 
In all these embryos, 79-89, as well as those in the preceding series, the 
tissues at first sight appear perfectly normal and healthy. They stain well, 
and one may find some nuclei in karyokinesis. But in one part or another 
may be seen, among the perfectly normal ones, numerous nuclei, in which 
the chromatin is crowded at one or both poles, leaving the centre perfectly 
clear. In those apparently just entering on the process, the chromatin stains 
very deeply, as in nuclei undergoing karyokinesis, but in the later phases 
the color becomes less intense. The nuclear wall disappears, and finally nothing 
is visible but a few faint, isolated chromatin granules, which in turn disappear 
also. Immense masses of these nuclei are sometimes seen at the anterior and 

