It is enclosed in a thin, structureless membrane, and is very opaque. The ova are laid in spherical, soft gelatinous capsules with no apparent orientation. Capsules produced by laboratory animals average about half an inch in diameter, and, as noted above, contain about two thousand eggs, although this number varies considerably. Cleavage is total and spiral, and often unequal. The micromeres are strikingly large and clear.
Gastrulation is apparently by epiboly.
In young forms, an opaque digestive tract can be seen, and beside it another dark mass which is probably unabsorbed yolk. A clear vesicle, which may represent a head kidney, is sometimes visible. A dark lobe, the liver, lies to the left of the stomach. An eyespot and a pair of statocysts develop. The natural history of Bulla hydatis Linn.