ON DEVELOPMENT OF ATYEPHIRA COMPRESSA. 415 
tures which appear to me of some interest. Up to the valuable 
contributions of Reichenbach almost nothing was known on 
the origin of the nervous system among the Decapod crus- 
taceans. 
The result of his investigation is briefly this ( x 5 . 2 t ~ 0 ' 59 ). 
The whole nervous system arises from (1) the median groove, 
(2) the lateral strings, and (3) the depressions in the cephalic 
lobes. The cells of the lateral strings and the groove give rise 
to the ventral cord, while those of the cephalic depressions 
become the supra-oesophageal ganglion. Some of my sections 
of the nauplius seem to show the similar structure. Thus the 
fig. 63, which is the transverse section of an embryo through 
the cephalic region, shows the thickening of the ectoderm cells 
on both sides of the median line (fig. 63, cd.). In figs. 64 
and 65, which represent two consecutive sections passing 
through the mouth opening, are shown the structures of the 
same nature. In fig. 66, a section cutting through the posterior 
part of the mandibles, the ectoderm is quite thick on both sides 
of the median line, which possibly corresponds to the lateral 
thickening of Reichenbach. Fig. 67, which represents a sec- 
tion similar to fig. 66, but of an embryo slightly older than it, 
shows two circular masses of cells on each side of the median 
line below the ectoderm cells. These appear to be the section 
of the two lateral portions of the ventral nerve-cord after its 
separation from the superficial ectoderm. But as these are the 
only sections by which I can get any knowledge of the origin 
of the nervous system, and as I have neither traced the origin 
nor the fate of the structures described, I have not written 
here anything more than a short description. 
The embryo gradually gets larger (fig. 68). New appen- 
dages (fig. 68, mx. 1, 2, and mxp. 1) are formed, behind the 
mandibles, in regular succession. The two maxillae (fig. 68, mx. 
1, 2) are at first single appendages like the mandible ( md .), but 
soon become bilobed. The appendages behind the maxillae 
are bilobed from the time of their appearance. The appearance 
of the maxillae as single-lobed appendages differs a little from 
the case ofPalaemonites ( 3 -A?), where they are bilobed from 
