46 



GL 0. SARS. 



[No. i. 



After the ova have remained a short time within the incub- 

 atory cavity, the egg-membrane, at first in imniediate contact with 

 the yolk, becomes separated from it, a clear fluid accumulatiug 

 between (see Pl. VI, figs. 1, 2), aud auother delicate membraue 

 is theu formed, investing the yolk. The egg thus becomes euvel- 

 oped by two membranes, the outer egg-membrane, or the chorion 

 (ch), and the inner, or the vitelline membrane (v). The segmenta- 

 tion-process of the yolk and the formation of the ectodermal cell- 

 layer, I have not been enabled to trace out fully; both these 

 processes, however, would seem to be performed in much the 

 same manner as in the Cladocera, the former being not, as in 

 other known Phyllopoda, total and regular, but superficial and 

 unequal, with the central part ot the yolk unsegmented till 

 the ectodermal cells have been formed. 



The earliest observed stage of the embryo is represented 

 Pl. VI, figs. 3 — 5. It exhibits in this stage a well-night globular 

 form, and is still invested with the two egg-membranes, the 

 inner (v) being now in part very distinctly perceptible. The 

 body is divided into two unequal portions, limited by a circular 

 instriction, the smaller constituting the cephalic part, or 

 head, the other the trunk. On turning round the embryo 

 so as to get a lateral aspect (fig. 3), the ventral side 

 appears much more strongly curved than the dorsal, the latter 

 being somewhat flattened and withdrawn from the vitelline mem- 

 brane (v)\ and inside this membrane, close to the dorsal surface 

 of the embryo, another very delicate membrane (l) can be traced, 

 — the embryonic cuticle, or larval coat. The clear outer 

 zone of the body, constituting the ectodermal cell-layer, is, 

 moreover, considerably broader on the ventral side than on 

 the dorsal, where it is almost obsolete. On the sides of the 

 head, somewhat nearer the ventral surface, two p^irs of bud-like 

 prominences are found to occur, the upper pair (a 2 ) the larger 

 and slightly bilobular at the tip, the lower pair (M ) quite simple 

 and rounded: the former represent the antennæ, the latter the 

 mandibles. No other appendages are as yet distinguishable, and of 

 the antennulæ, which in other Crustacea generally appear simul- 



