248 B. NATURAL HISTORY. 
line the whole embryo is raised above the surface of the yolk, 
being quite distinct from it, and of a paler hue. In more advanced 
eggs three pairs of rudimentary limbs were observed, the most 
anterior pair representing the first pair of limbs (false mandibles 
of Savigny) being much smaller than the others. The mouth 
Fig. 1. Fig. 2. 
Embryo of Limulus. 
opening is situated just behind them. In asucceeding stage (fig. 1, 
ar, areola; am, inner egg membrane ; ch, chorion) the embryo forms 
an oval area, surrounded by a paler colored areola, which is raised 
into a slight ridge. This areola is destined to be the edge of the 
body, or line between the ventral and dorsal sides of the animal. 
There are six pairs of appendages, forming elongated tubercles, 
increasing in size from the head backwards; the mouth is situated 
between the anterior pair. The whole embryo covers but about a 
third of that portion of the yolk in sight. At this time the inner 
egg membrane (blastoderm-skin ?) was first detected. 
The outer membrane, or chorion, is structureless ; when ruptured, 
the torn edges show that it is composed of five or six layers of a 
structureless membrane, varying in thickness. The inner egg 
membrane is free from the chorion, though it is in contact with it. 
Seen in profile it consists of minute cells which project out, so 
that the surface appears to be finely granulated. But on a vertical 
view it is composed of irregularly hexagonal cells, sometimes five- 
sided, and rarely four-sided, hardly two cells being alike. The 
walls of the cells appear double, and are either strongly waved, or 
have from three to five long slender projections, with the ends 
sometimes knobbed, directed inwards. These cells are either 
packed closely together, or separated by quite a wide interspace. 
In a subsequent stage (fig. 2) the oval body of the embryo has 
increased in size. The segments of the cephalothorax are indica- 
ted, and the legs have grown in length, and are doubled on them- 
e 
