1. Histological studies on the developing retina in mice, rats and Irish setters affected with a hereditary retinal degeneration of the retinitis pigmentosa type do not lend any support to the view that the retina is fully developed before degenerative changes set in — as postulated by the conception of abiotrophy. In fact, the retina in these animals, though reaching functional maturity, lacks the final histological differentiation that leads to fully developed rods: the rods are present, but they are rudimentary and do not survive for the normal span of life. These studies suggest that the abiotrophies (or heredo-degenerative diseases as they are sometimes called) are in fact the mildest of congenital defects and do not therefore stand in sharp contrast to congenital abnormalities.

2. Clinical studies on the hereditary affections of the retina show that the distinction between congenital non-progressive disorders and abiotrophic progressive disorders is not valid. Most congenital abnormalities of the retina such as macular cyst, retinal aplasia, asymptomatic macular defect and congenital sex-linked detachment, show a considerable post-natal course. In fact it is difficult to find a clear example of a congenital non-progressive retinal anomaly. In the choroid, the one substantial congenital defect — macular coloboma — is non-progressive.

3. By definition all abiotrophic defects have a progressive course. This, however, varies considerably in the different affections. In the retina, retinitis pigmentosa represents not one disease but a whole series of affections. This is seen from such well differentiated types as the recessive, dominant, recessive sex-linked and intermediate sex-linked varieties. Furthermore, in different families there are different associated anomalies, such as glaucoma, cataract, ophthalmoplegia or macular dystrophy. Unilateral retinitis pigmentosa appears to be a somatic mutation. The macular dystrophies are likewise a group rather than one disease, as shown by the different genetic behaviour and the different clinical features of the recessive, dominant and sex-linked varieties of macular dystrophy. Furthermore the ophthalmoscopic reaction ranges in different families from fine granular changes to fairly heavy pigmentary changes, or whitish lesions of the exudative type. In the choroid, choroideremia and choroidal sclerosis — with its generalized, central and peripapillary types — are the two outstanding abiotrophic affections, with markedly dissimilar clinical behaviour.

4. An important abiotrophic affection coming on in middle age is dominant generalized fundus dystrophy with its stormy onset, oedematous and exudative reactions at the macula, and ultimately extensive atrophy throughout the fundus. This has to be distinguished from another dominant affection with a much milder course — Doyne's choroiditis. The choroidal nature of these two affections is not definitely established. They may possibly both be disorders of the membrane of Bruch.

5. Retinoblastoma appears to occur in both a genetic and non-genetic variety. It is likely that the bilateral case represents a germinal mutation and that the unilateral case is generally a somatic mutation; only the germinal mutation is, of course, transmitted.

6. A number of other affections may have a genetic background as yet ill-established.