No.1.] VARIATIONS IN LIMULUS POLYPHEMUS. 35 
abdomen is perhaps merely retarded and may appear later in 
its normal condition. This is probably the case with Figs. 9, 
10, and 15. 
The TLelopore.—In the normal embryos, at a very early 
stage, the posterior end of the body, or the anal plate, consists 
of a broad mass of proliferating cells, representing a modifica- 
tion of the posterior of the two primitive cumuli that constitute 
the beginning of the embryo. 
In Fig. 1, there is already formed a primitive streak-like 
invagination in the anal plate, representing a specialization of 
this proliferating area. Along this proliferating furrow, ecto- 
derm and inner-layer cells become continuous. Without fol- 
lowing out its history in further detail here, it is enough to 
state that it persists, either as a solid mass of cells or as a 
furrow, up to stage C, after which it gradually disappears. The 
anus does not appear on the flat abdominal plate till about 
stage £, Fig. 7, long after the primitive streak has disappeared. 
But these normal conditions are frequently deviated from, 
giving rise to a great range of variations. 
[n many cases when the abdomen ts abbreviated or absent, the 
entire remaining region, where the abdomen should be formed, ts 
tnvaginated to form a deep depression, varying greatly in size 
and general appearance. ‘This depression, or ¢elopfore, is not a 
modification of the anus or of the primitive streak, for one or 
the other of them may, in some cases, be found at the bottom 
of the depression. It seems to be the result of the active pro- 
liferation that is going on in this region during the formation 
of new segments. 
A common form of the telopore is shown in Figs. 9, 10, and 
fiemanotieran Migs), 16, 19, 21, 22, 23) 24, 36; 50,62, 07,69. 
In Figs. 20, 21, 23, 51, the whole abdominal region is so deeply 
depressed that the posterior thoracic and abdominal appendages, 
when they are developed, are carried into the cavity. 
At the bottom of the depression, I have often found in sec- 
tions the proliferating primitive streak, not markedly different 
from that in the normal position. 
The anus may finally appear at the bottom of the telopore, 
after the primitive streak has disappeared. In some cases, it 
