After this time, many die from starvation or because they become trapped and held in the surface film. The fertilized, uncleaved egg is small, having a diameter of about 80 microns. 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.