1. Observations under filtered ultra-violet light of the available material of Microsporon audouini, M. felineum, Trichophyton gypseum, T. violaceum, T. album and Achorion schoenleini indicate that the species belonging to the genera Microsporon and Achorion produce an intense green fluorescence in the substance of hairs infected by them. Trichophyton-infected hairs appear a paler bluish-white. Normal pigmented hairs appear dark.2. It is shown that the spores and hyphae of the fungus whether within or without the hair are relatively non-fluorescent as contrasted with the substance of the infected hair.3. A fluorescent substance present in hairs infected by M. audouini, M. felineum and Achorion schoenleini is readily soluble in hot water. No such substance could be extracted from the hairs infected by Trichophyton sp., or from normal hairs. It is therefore believed that the presence or absence of the green fluorescence is pathognomonic.

Studies were undertaken to correlate structure with life cycle in four common species of fungi causing skin disease in man. Hairs naturally infected by Microsporon audouini, M. felineum, and Trichophyton gypseum and scutula of Achorion schoenleini were placed, without any nutrient medium, in van Tieghem cells at various humidities controlled by osmotic solutions of known vapor pressure. These fungi, which while parasitizing the animal body produce hyphae and thallospores only, were then found to undergo a second period of growth on the detached infected host tissue in moist atmospheres. During this phase they produced all the highly differentiated spore forms previously known only on various media. The aerial hyphae, aleuriospores, fuseaux and spirals produced by such in situ cultures are described in some detail. It is suggested that in nature the saprophytic phase is initiated when the infected tissues fall from the animal body in a moist situation, and that this phase may have significance in the epidemiology of the disease.

Hyphal fusions have been recognized as an important character of dermatophytes.In Microsporon audouini, M. lanosum, and Trichophyton gypseum hyphal fusions: (1) are formed between hyphae of one and the same mycelium isolated from a single patient, (2) are formed between any two mycelia of the same species isolated from two different patients, and (3) are not formed between a mycelium of one species and a mycelium of another species.The occurrence or non-occurrence of hyphal fusions between hyphae of two mycelia of different origin may be applied as a criterion for identifying species of dermatophytes whose specific nature is uncertain.

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