Pigment of tail-fin melanophores in periodic albino Xenopus laevis tadpoles is dispersed in response to darkness and to alpha-MSH in a manner similar to wild-type melanophores. However, periodic albino tadpoles lack the response to different background conditions and the melatonin-induced aggregation in darkness. The tyrosinase activity in cells of the latter type tadpoles is weak compared to the wild-type cells. Ultrastructural examination of melanophores from periodic albino mutants and cells from wild-type tadpoles shows similar organelles at corresponding sites. A morphological difference can be observed in the fine structure of the melanosomes, which in albinos resembles an earlier stage of development. It is postulated that periodic albino Xenopus laevis possess the cellular mechanism to disperse pigment in the melanophores, but that under physiological conditions the release of alpha-MSH appears to be absent or scarce.

Electron microscopic observations confirm the presence of optic terminals of retinal origin in the preoptic area of Rana temporaria. After unilateral section of the optic nerve, degenerating axon terminals were observed among the neurosecretory cells of the preoptic nucleus, both ipsilaterally and contralaterally. The retino-preoptic terminals apparently establish axo-dendritic synapses with non-neurosecretory neurons. Many more degenerating fibres were seen in the ipsilateral preoptic nucleus than in the contralateral nucleus. These are presumed to pass through and to cross in the dorsal posterior part of the optic chiasm.

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