No.1.] VARIATIONS IN LIMULUS POLYPHEMUS. E3t 
It is very interesting to note that these two cell masses resemble in position 
and general appearance the two proliferating areas, “ primitive cumuli,” which at 
a very early period give rise to the head and trunk of normal embryos. 
No trace of primitive cumuli is visible in normal embryos after stage C, and 
as these embryos are certainly as far advanced as that stage, they either must 
have retained to this late period a very early embryonic character, or else the 
processes of degeneration have carried them back a second time to their original 
condition. In either case, the facts seem to emphasize the morphological impor- 
tance of these two proliferating centres. 
There is no very reliable indication as to what is the head or tail Ax/age, 
except perhaps in Fig. 73. The embryos have been oriented in each case with 
the smaller An/age at the anterior end, and arranged and numbered as far as 
possible in accordance with the degree of degeneration. 
It is obvious that such a plan of arrangement cannot be followed with any 
accuracy, since each embryo has its own way of degenerating, begins to degenerate 
at different stages of development, and may have been defective at the outset. 
What finally happens to these degenerated embryos cannot be determined, since 
further degeneration of either head or tail 4z/age would make it impossible to 
determine whether it belonged to this series or the next. But I see no reason to 
doubt that degeneration goes on till even the head and tail Ax/agen are ab- 
sorbed and disappear completely. 
While Fig. 73 is the first one of the series here represented, numerous inter- 
mediate stages between it and normal embryos of stages C and D were found. 
Just how far normal] development may proceed before this general degeneration 
begins could not be determined. I do not recollect seeing any embryo undergoing 
this phase of degeneration, which in whole or in any part had reached a higher 
stage of development than that characteristic of stage D. But it must be borne 
in mind that all these embryos had been developing or degenerating in the same 
jars with normal eggs, for from fifteen to eighteen weeks. The normal ones 
passed through their whole development and escaped from the jars as trilobite 
larvae, in but little more than half this time. 
FIG. 72, X 60, sectioned. In this embryo, every trace of cephalic lobes, 
oesophagus, nerve-cords and appendages has disappeared. The mesodermic area 
is very large and irregular. Anteriorly the margin of the same is pretty close to 
the axis of the embryo. It there consists of numerous star-shaped masses of cells, 
densely crowded together, m.a. The posterior margins are much thickened 
to form two great diverging arms, which are united at their distal extremities, but 
are separated medianly by a triangular area devoid of mesoderm cells (compare 
Fig. 67). These thickened arms unquestionably represent the posterior margins 
of the mesodermic area which has not completely concresced along the median 
line. It is important to recognize this, because it shows approximately to what a 
late stage of development this embryo belongs. Such embryos as this and those 
that follow on this plate, cannot for a moment be considered as belated normal 
embryos, in early stages of development. Their great age (two or three months), 
the concrescence of the mesodermic margins, the histological character of the 
tissues, and a comparison with the early stages of normal embryos, is sufficient to 
prove this beyond any question. 
A thin layer of ectoderm covers the greater part of the mesodermic area, but 
it is thickened and elevated in the axial region, over what probably represents the 
