No. 1.] VARIATIONS IN LIMULUS POLYPHEMUS. EEL 
Back of the tail lobe is the usual ectodermic thickening, f.a.c., and beneath it 
is the usual post-anal cloud of mesoderm, formed by the concrescence of the 
margin of the mesodermic areas. 
FIG. 19, X 60, not sectioned. Embryo not depressed. Thoracic appendages 
show a common, but abnormal mode of growth. Abdomen absent. Cephalic 
lobes slightly convex, with very thin ectodermic fold, probably consisting of a 
single layer of cells growing over their lateral portions. Marginal fold knotted 
and conspicuous posteriorly, but not continuous across the median line. No in- 
vagination of anal plate. Mouth very long and narrow. Mesodermic area, com- 
paratively small, circular, and with conspicuous rim, m.a., especially at sides. No 
conspicuous mass of cells due to concrescence of mesodermic rim back of anal 
plate. 
FIG. 20, X 60, sectioned. Embryo short and narrow and deeply depressed. 
Marginal fold sharply defined. Cephalic lobes shortened and partly concealed by 
a broad overhanging ganglionic fold. 
Abdominal plate without appendages and sharply infolded to form a very deep 
oval invagination, from the walls of which arises a cloud of degenerating yolk 
cells. 
The mesodermic area is extensive. It rim is well defined, and posteriorly 
shows very beautifully its mode of concrescing. In this case the outlines of the 
posterior, mesoblastic segments are very clearly shown. 
FIG. 21, X 60, sectioned. Chelicerae and abdomen absent. Large open 
depression on margin of cephalic lobes, bounded on the sides by a prominent, 
overhanging lip. Mouth very large, with rostrum-like projection in front. Body 
of embryo depressed and surrounded by prominent, knotted, marginal fold, which 
is discontinuous posteriorly. 
_A deep and broad invagination of the anal plate has carried the last pair of 
thoracic appendages inwards till they project from its sides. A conspicuous 
elevation back of anal plate, Z.a.c., due to the concrescence of the posterior 
margin of the mesodermic area. The lateral and anterior portions of the rim 
are not represented. 
Fic. 22, X 60. A very common form of embryo in stage Y. Not sectioned. 
Differs from the normal in its shortened, compact shape, and in being deeply 
sunken in the yolk. It was outlined to show a rather frequent modification of 
the direction in which the thoracic appendages grow, the anterior pairs pointing 
backwards and laterally and the posterior ones forward and inwards. It is 
interesting because it occurs frequently, but especially because it is between the 
third and fourth thoracic metameres that transverse division sometimes takes 
place. The difference in the disposition of the appendages recalls a very similar 
one that obtains between the mouth parts and the walking appendages of insect 
embryos. 
Fic. 23, X 60, not sectioned. Embryo narrow and depressed. Appendages 
normal except in the separation of third and fourth pairs, a rather frequent occur- 
rence, that seems to have some connection with the transverse fission that often 
occurs here, and with the difference in direction between the appendages in front 
of and behind this space. Two pairs of abdominal appendages present, but they 
are carried inwards by the invagination of the anal plate, so that they project 
from the sides of the cavity. Marginal fold prominent, especially posteriorly, and 
continuous anteriorly with a well-defined fold overhanging the invagination on 
the sides of the cephalic lobes. Mesodermic area not conspicuous or well defined. 
