724 General Notes, [July, 
The abdomen has not yet undergone segmentation; the incip- 
ient steps are represented in Fig. 2, where there appear to be 
arising five mesoblastic segments (1, 2, 3, 4,5). Between the first 
and second mesoblastic mass is a narrow cavity which sends a 
branch forward to the base of the abdomen, and a second 
obliquely downward and inward ; at 2 and 3 in Fig. 2 there are 
narrow cavities or splits (somatic cavities ?) which communicate 
with a longitudinal internal opening, which extends in a direction 
parallel to the under (now outer) surface of the abdomen. In 
this respect the embryo of Limulus is very different from that of 
the scorpion and spiders (see especially Balfour’s Figs. 5, 6, Pl. 
xix, and Fig. 15, Pl. xx), where the abdominal segments, with 
their appendages and somatic cavities are formed contemporane- 
ously with those of the cephalothorax. The innermost meso- 
dermic cells are now arranged in long cords, destined to form the 
ventral adductor muscles of the abdomen. 
The mode of formation of the head and its shape at this time 
presents important differences from that of tracheate embryos. 
The procephalic lobes are not developed ; the preoral portions of 
e head, z. e., that part in front of the first pair of limbs is very 
small, short and narrow, merely forming the end of the oval blas- 
todermic disc, seen in my earlier published figures. The struc- 
ture of the preoral portion of the head (procephalum as we may 
term it), is seen in longitudinal section in Fig. 3, fc, to apparently 
consist merely of an extension of the postoral part of the head ; 
with apparently one or two splits in the mesoderm (mst, ms*), t 
nature of which I do not understand; undoubtedly farther sec- 
tions and comparisons will throw light upon it. 
The first nervous ganglion is seen at Fig. 5 to result (as also 
first shown by Kingsley) in an ingrowth of the epiblast (nv. c) ; 
carrying into the interior a mass of epiblastic nuclei, which 
envelop the myeloid substance (my), which, as in older embryos, 
remains unstained by the carmine. 
The mesoblastic nuclei stop at a large cell (c), beyond which 
are long incipient loose muscle-cells with a few scattered nuclei. 
d procephalum terminates abruptly, forming, as seen in our 
saia figures already referred to, the end of the blastodermic 
sc. 
The absence of the procephalic lobes in the embryo Limulus 
of this stage seems to us to be a very significant fact, and to point 
- commi ith eate os. At the same time the gen- 
: eral mode of formation of the blastodermic disc (ventral plate) of 
