Nott) “VARIATIONS LNOEIMOLGS: POLYPHEM GS. £35 
posterior ends of the degenerating embryos. They are also seen along the 
longitudinal, median line, in the nerve-cords, and in the appendages. 
I have described and figured just such nuclei as these in the ectoderm about 
the eyes of Acilius, and have seen them in the degenerating masses of endoderm 
cells, or yolk nuclei, at the anterior and the posterior ends of healthy and normal 
scorpion, Limulus, and insect embryos. 
Similar nuclei are found, and they undergo the same changes, in the degener- 
ating amnion and serosa of insects, where these membranes, after forming a 
“dorsal organ,” are invaginated into the yolk. 
In none of these cases are the degenerating nuclei ingested by more vigorous 
amoeboid cells; on the contrary they disintegrate and then fade out of sight, 
probably by forming solutions that diffuse gradually throughout the yolk or other 
tissues. 
FIG. 79, X 60, sectioned. The mesodermic area is ovoid. There is a well- 
marked marginal thickening; also a post-anal one, /.a.c., showing where con- 
crescence has taken place. Along the axis of the embryo the thickened ectoderm 
is elevated and contains two distinct, elongated invaginations. At the posterior 
end of the second ore is a small and still deeper invagination. The lateral walls 
of the second invagination are constricted at a point that probably marks the line 
of separation between two adjacent segments; compare Figs. 61 and 81. The 
invaginated ectoderm, here, as in the preceding figures, probably represents the 
remnant of the whole axial ectoderm of normal embryos, z.e. cephalic lobes, nerve- 
cords and appendages, as far laterally as the marginal fold. 
This embryo may represent the last stages of an hour-glass embryo, the 
anterior sac being the remnant of the cephalic lobes and first three pairs of 
appendages, and the posterior sac the remnants of the last three thoracic 
appendages and the abdomen. Compare Figs. 52, 53, 54, 55, and 57. 
Fic. 80, X 60, sectioned. This is similar to the preceding, only the con- 
crescence of the posterior margins of the mesodermic area is much more clearly 
shown, /.a.c. There is hardly any trace of the axial portion of the embryo, other 
than an elongated, ectodermic thickening with a large diamond-shaped depression 
extending along it. In surface views, there are no indications of nerve-cords or 
appendages, but in sections there are slight differentiations of the thick wall of 
the furrow, that may represent the last traces of the nerve-cords and appendages. 
Fic. 81, X 60, sectioned. Over a heart-shaped, mesodermic area lies a thin layer 
of ectoderm, elevated considerably and thickened along the median line. Along the 
summit of the ridge is a series of depressions increasing in size from before back- 
wards. The third depression is so deep as to be almost tubular. The last is 
broad and shallow, but has a rather deep depression in its centre. Each depres- 
sion probably represents a segment from which nerve-cord, appendages, etc., have 
disappeared. The ectoderm has the appearance of old specialized tissue, although 
one cannot identify any particular kind of tissue or organ in it. 
Fic. 82, X 60, sectioned. This extremely instructive specimen shows clearly 
the real stage of development, and the nature of the changes this class of 
embryos has undergone. The mesodermic area is kidney-shaped, and precisely 
defined by a thickened margin. The latter forms two distinct lobes that are 
manifestly growing toward each other in order to concresce along the posterior 
median line. On examination of the normal embryos it will be seen that this 
phase of concrescence is not due till stage C-D. The abdomen of this embryo 
