112 PAW TEV: 
FIG. 24, X 60, sectioned. In this embryo the cephalic lobes and first two tho- 
racic segments have disappeared. The marginal folds have closed in front of the 
third pair of appendages, and the nerve-cord terminates abruptly just back of 
them. The abdomen is absent, and the marginal folds are contracted so as to 
extend across the median line just behind the sixth pair of thoracic appendages. 
In the median line, the fold is interrupted by a deep, thick-walled invagination 
with an oblong lumen. 
The invagination dips deeply into the yolk, and from its thick walls arise 
numerous nuclei which are seen scattered about in the neighboring yolk. The 
yolk nuclei are most numerous back of the invagination. An ectodermic thick- 
ening forms the broad, dark, post-anal band seen in surface views, 7.a.c. At its 
posterior end, the band becomes continuous with the thickened rim of the meso- 
blastic area. The post-anal cloud of yolk cells is formed in part by cells that 
have migrated from the walls of the invagination, and in part by those arising 
from the thickened mesodermic rims, which have concresced along that line. 
A large marginal vesicle, m.v., is seen in the right anterior margin of the 
mesodermic area. 
Fic. 25, X 60, sectioned. A remarkable embryo in stage C, in which the 
cephalic lobes are reduced to a flat circular plate, slightly depressed, so that it is 
surrounded on all sides by a vertical wall. Near the centre of the disc, which is 
separated by a considerable distance from the remainder of the embryo, is a small 
pit representing the oesophagus, and in the yolk below the disc is a great, irregular 
mass of cells, with numerous pseudopodia-like streamers of cells, extending still 
deeper into the yolk. 
The body of the embryo, which consists of three appendage-bearing segments, 
is bent into the yolk at the posterior end. Back of this abbreviated trunk is a 
broad depression bounded on either side by steep walls, which gradually shallow 
posteriorly to the surface of the ovum. 
Fic. 26, X 60, sectioned. A very compact embryo of stage Cand YD. All the 
ectodermic layers are very thick, and the mesodermic area is constricted so that 
its peripheral margin lies close around the body of the embryo. The greater part 
of the mesoderm forms two thick bands on either side of the body, close to the 
median line. 
The cephalic lobes are completely covered by a hood-like fold extending back 
almost to the second pair of thoracic appendages. At the bottom of the cephalic 
cavity is a small pit, the histological character of whose walls indicates that it is 
the oesophagus. 
Surface of embryo covered with bacteria. 
FIG. 27, X 60, sectioned. Very much shortened embryo in stage C. 
The rudimentary cephalic lobes are covered by a backwardly directed fold, and 
only three pairs of appendages, probably the fourth, fifth, and sixth, are represented. 
What looked in surface views like an anus was present, but its nature could not 
be determined in sections. Such embryos as those in Figs. 26 and 27 are rather 
common, and possibly they were seen by Dohrn and Osborn, and gave rise to the 
statement that Limulus passes through a nauplius stage. 
Fic. 28, X 33, not sectioned. An embryo in stage C. 
It consists of an abdominal plate and four metameres, probably representing 
the last three thoracic and first abdominal ones. The appendages are separated 
by a very wide space, over which the neuromeres (?) and mesoblastic somites 
extend as long, narrow bands. 
