Changes in the size of germ cell nuclei during the development of the testis of the ram and rat.

The surface areas of the plane of maximum size of germ cell nuclei have been determined at different stages of development of the testis of the ram and rat. The distributions of these surface areas both in the developing and the adult testis were found to be logarithmically rather than arithmetically normal. A basic pattern of change in surface area takes place during the development of the testis in both species of animal. The overall pattern of change consists of nuclear enlargement prior to the onset of spermatogenesis. By the time spermatogenesis has become fully established, germ cell nuclei have diminished in size. The enlargement in the ram testis does not take place until just before the onset of spermatogenesis. In the rat it is obvious during foetal life and continues until the gono- cytes begin to divide at the end of the first week of post-natal life. The enlargement is not, as was believed by previous workers, indicative of degenerative changes taking place in these cells, but is part of the normal differentiation of gonocytes. Abnormally enlarged germ cell nuclei are found in the ram in mid-foetal and particularly in early post-natal life prior to the appearance of prospermatogonia. Excessively enlarged nuclei are also found in the rat in the second week of post-natal life after the gonocytes have begun to divide. The reasons for the abnormal enlargement in both species of animal require further investigation. In both the ram and the rat, the nuclei of type A spermatogonia increase in size prior to dividing to initiate a new spermatogenic cycle.