Age, 10.9 days.

Standard length, 3.87 mm. Length of caudal fin, 0.47 mm.

Mesencephalon width, 1.03 mm. Depth of yolk, 1.39 mm.

The anterior margin of the pericardial serosa forms a small circle with the mouth and the front of the head protruding (fig. 36A, B).

The number of melanophores has increased considerably, accompanied by the first appearance of corolla-type melanophores on the head (fig. 37A). Since these were present in all the strains of platyfish, sword-tails, and hybrids studied, they may be identified as the micromelanophores. The intestine possesses a single complete coil. The urinary bladder is large, bilobed, and opens to the exterior by a slit-shaped urinary pore. The mesonephric ducts com-municate with the bladder at its most posterior margin, near the urinary pore (fig. 37B).