350 W. F. BR. WELDON. 
The embryo drawn in fig. 7 corresponds to Reichenbach’s 
stage EF, figured on pl. iii, fig. 8, of his work. The optic lobes 
have much the same appearance as that already seen in fig. 6, 
while the bands connecting these with the ventral plates have 
made considerable progress. The first antennz are already 
visible as well-marked hemispherical projections, while a 
ganglionic rudiment appears as an aggregation of nuclei at the 
base of each. The second antenne are indicated, behind the 
first, by a slight increase in the crowding of the nuclei, which 
in this place exhibit to a marked degree the curious arrange- 
ment in intersecting curves which is so well seen in Reichen- 
bach’s figures of Astacus. Behind the second antenne, and 
just in front of the ventral plates, the mandibles are already 
present as a pair of slight projections. The concavity of the 
ventral plates is well marked at this stage, and causes these 
‘structures to appear separated from one another in the middle 
line by a raised ventral crest. A quite similar horizontal ridge 
seems to bound the ventral plates anteriorly; and this ridge 
has apparently been mistaken by Kingsley for the commence- 
ment of a pit, which will ultimately, according to him, produce 
the ventral flexure of the body characteristic of the later stages. 
Kingsley appears to have examined a section passing through 
the blastoporic pit (which he calls “‘ proctodeum ”) and through 
the concavity of one ventral plate, which he has regarded as 
representing the commencement of an abdominal flexure; but 
his figures are very difficult to reconcile with the appearances 
seen in sections by myself. © 
Between the first antenne is a median aggregation of 
nuclei; and behind this, on a level with the posterior border 
of the antennules, is a small depression, which is distinctly 
visible in sections, but which could not be satisfactorily shown 
in fig. 7. This depression is the mouth ; and from its position 
in this and in the next stage (compare fig. 8) the first antenne 
are evidently preoral from the very earliest period at which 
the mouth is visible. This view of the relations between the 
mouth and the first antenne is, I think, unquestionably 
justified by the figures referred to, together with those of the 
