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Prof. E. W. MacBride.
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FIG. 2. Diagrammatic Longitudinal Frontal Section of a Larva about 18 days old.
ad., ce., st., stom., as before ; EcJi., invaginated ectoderm about to form the oral disc of the future Echinus ; hy., hydroccele ; l.a.c., left anterior coelom (ampulla of stone-canal) ; l.p.c., left posterior coelom ; m.v., madreporic vesicle (right hydro- ccele this is not in the plane of the figure) ; r.a.c., right anterior coelom ; r.p.c., right posterior coelom.
st.c.
FIG-. 3. Diagrammatic Transverse Section of a Larva about 18 days"old.
Names as before ; in addition, int., intestine; m.p., primary madreporic pore;
p.c., pore canal.