By means of semithin section histology and electron microscopy the following male germ cells are described: gonocytes and prospermatogonia in the embryonic and fetal testes; spermatogonia and primary spermatocytes in the testis of the adult; abnormal spermatogonia and spermatocytes, which are not tumour cells. In contrast to these cells, four main tumour cell types which on the basis of morphological criteria can be said to be derived from germ cells, are classified and designated as Tc 1 to Tc 4. The various tissues accompanying the tumour cells are briefly described.