No.1.] VARIATIONS IN LIMULUS POLYPHEMUS. 137 
to form a marginal vesicle, #.v. In the centre is a kite-shaped sac opening to the 
surface by an elongated figure-8-shaped opening,c. The anterior part of the floor 
of the sac protrudes upwards and forwards (cephalic lobes[?]), and from this 
inclined wall projects forwards a short tube or stomodaeum(?), 6. Both structures 
are seen in sections in Pl. X, Fig. 85',?.. The sac extends backwards some distance 
beyond the aperture, c, as a cylindrical tube which ends blindly deep in the yolk. 
There is a shield-shaped thickening of the ectoderm, lying over and a little to 
the right of the sac. It is represented by the shaded area, d. A section through 
this thickening and the posterior end of the sac is shown in Fig. 857. 
Fic. 86, X 60. This embryo is similar to the preceding one. Unfortunately, it 
was lost or injured before it could be sectioned. In the centre of the mesodermic 
area is a thick-walled sac, which communicates with the exterior by a diamond- 
shaped opening. The anterior end of the sac is very broad and spacious, and the 
posterior part a deep, nearly closed furrow terminating in a tube that dips vertically 
into the yolk. The relation of these parts, so far as they could be determined 
in surface views, is shown in the accompanying median, longitudinal, optical 
section. Fig. 86, 2. . 
Fic. 87, X 60, not sectioned. The mesodermic area appears to be nearly 
circular, but its outline is very indistinct. In its centre is a conical, thick-walled 
projection, with an opening at the summit leading into a large, nearly spherical 
sac. Specimens resembling this one in all but unimportant details are comparatively 
common. They are evidently further modifications of the conditions seen in the 
five preceding figures. This specimen was not sectioned, as it did not appear to 
present any noteworthy features. 
Fic. 88, X 60, sectioned. In this case a sac like that seen in the preceding 
figures is beginning to break up. The mesodermic area is distorted, and on the 
left ill defined; on the right the rim of the mesodermic area shows the character- 
istic, star-shaped masses of cells that are spreading out into the yolk, preparatory 
to their final dissolution. 
In sections of other specimens, apparently in a similar condition to this one, 
the walls of the sac have lost their sharp outlines, as though they were gradually 
falling apart ; and the cells composing them have the character of closely packed, 
lymphoid cells, rather than that of the columnar ones seen in the preceding figures. 
Still one would hardly suspect, from a mere inspection of the sections, that these 
specimens were anything else than very early stages of normal embryos. 
Fic. 89, X 60, sectioned. In this remarkable specimen we have a good 
example of the last stages in the degeneration of the embryo. In all the other 
cases I have studied, where the thick walls of the sac had broken up into a form- 
less mass of cells, the latter were usually rounded, lymphoid ones, showing no 
further differentiation. But in this instance the entire mass, representing all that 
is left of the embryo, is composed of those peculiar cells, containing coiled filaments 
that are only found at a late embryonic period in the thickened margin of the 
embryonic area. I have already referred to these cells in one of my earlier papers 
(“Origin of Vertebrates from Arachnids,” p. 375), and at that time supposed they 
were purely embryonic, and gave rise in some instances to the future muscle cells. 
Since then, however, I have found them as free, amoeboid cells, in great 
numbers in the tissues of the adult animals. They differ somewhat in histological 
characters from the embryonic ones of the mesodermic margin, but there can be 
no question whatever about their identity. In the adult these cells are very 
