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Sixteen cases of pigeons without a discoverable trace of gonadal tissue have been found and described. Supplementary groups, one of fifteen birds in which one gonad only was abnormally absent, and still another of seventeen cases of nearly suppressed gonads, comprise the principal material obtained and examined. It is practically certain that this absence of gonads was complete and persistent--not a temporary condition nor one attained shortly before examination of the birds. Previous observations on the "no gonad" and "one gonad" condition in birds and in other animals are reviewed. The basis of the gonadless condition is a purely developmental one, in which it is probable that physiological rather than cytological or genetic disturbance is chiefly concerned. An attempt is made to identify the type of disturbance involved in this developmental anomaly. It is concluded that such failures of gonad development are a consequence of the failure of all "primordial germ cells" to reach or enter the anlage of the germinal ridge. Some data are available for a test of this view; they are found to be in conformity with it. Despite the complete and persistent absence of testicular tissue, some of these birds developed complete and emphatic masculine behaviour; some additional secondary sex characters observable in these forms were likewise developed to one or another degree. Since such birds were, according to all the evidence, entirely without interstitial or spermatogenic tissues, it is clear that the thing which is normally accomplished with the aid of the testis incretion may also be accomplished without it. In such cases it is considered that the effective impulse in the determination of the individual's sex, is capable of effecting the development of at least certain of these "secondary" sex characters. This material offered no opportunity to examine the effects of loss of gonad incretions on plumage characters. These results, like those of other parts of our studies, lead to the view that the gonad incretions are "controlled" as well as controlling factors in sex development. That they, like the chromosomal or genetic sex factors, rest upon the more basic condition which we elsewhere identify as metabolic level or rate.

Birds Without Gonads: Their Origin, Behaviour, and Bearing On the Theory of the Internal Secretion of the Testis