GEGENBAUR, ON SAGITTA. 53 
Viewed from the surface, the external opening of the canal 
—the future mouth, since the whole cavity becomes the 
intestinal canal—appears in the ova of Sagitta bipunctata, 
as a round, funnel-shaped depression, whilst in the ova of 
the smaller species it appears to be more elongated trans- 
versely. 
The whole history of comparative deveiopment offers 
nothing analogous to this surprising mode of formation of 
the rudimentary intestine out of a central cavity which 
makes its appearance in the earliest stages of segmentation 
of the vitellus ; and in this respect, again, Sagztta appears to 
constitute a paradoxical form. 
When this canal and the central cavity into which it leads, 
and which increases in size and becomes irregular in form, 
have been fully developed, the embryo still completely fills 
the vitelline membrane, by which it is closely invested on 
all sides, excepting at the spot where the mouth is situated, 
and where the surface of the body presents a shallow de- 
pression. 
The two layers of cells, of which alone the body of the 
embryo is, up to this time, constituted, are now broken up by 
a further transverse division of the individual cells, so that 
more rounded embryo cells soon become visible. — 
In consequence of an increase in the length of the hitherto 
spherical embryo, the body now necessarily becomes curved ; 
a change which indicates a new and not less characteristic 
stage. There now takes place a farther differentiation of the 
layers of cells which were apparent at a former period. Of 
the cells produced from the single internal layer a stratum is 
formed whose clear and minute elements immediately sur- 
round the intestinal canal, and are clearly distinguishable on 
the outer aspect from the peripheral layer of cells formed from 
the simple outer layer. These layers, therefore, composed of 
numerous superimposed cells, correspond each with one of 
the primitive strata, which, we have seen, were derived from 
a transverse division of the simple pyramidal cells. 
The central stratum Gegenbaur regards as the rudiment 
of the intestinal wall, and in the peripheral he recognises the 
integument of the body. 
The anterior and posterior ends of the body approach each 
other, so that the mouth comes to lie within the point of 
incurvation. The convex surface of the embryo, therefore, 
corresponds to the dorsal aspect. Of the further changes 
in the embryo, nothing precise seems to have been observed. 
On the ninth or tenth day the animal is completely formed, 
and begins to manifest its maturity by struggling move- 
