Abstract
In a previous communication (Grüneberg 1938), a new recessive lethal mutation has been described in the rat which produces a variety of ano­malies in various parts of the body. It was shown that all these deviations from the normal, including those disturbances which lead to the death of the lethals, are ultimately caused by an anomaly of the cartilage. All the other manifestations of the gene are therefore of a secondary nature. The histology of the abnormal cartilage will be described in the first part of this paper. For the anomaly of the cartilage, no obvious cause could be discovered by morphological means. It was pointed out, however, that this does not necessarily mean that the gene acts primarily on the cartilage. There remained the possibility that the cartilage itself was only secondarily affected by some general physiological condition of the body which pro­duced no other visible changes.

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