A comparison has been made between gonocytes and adult type undifferentiated spermatogonia in the mouse. The following morphological resemblances were noted: 1. Proliferating gonocytes, at days 13 and 14 p.c., resemble proliferating undifferentiated spermatogonia between stages IX and IV in the adult. 2. Gonocytes in G1 arrest, from day 15 p.c. until birth, resemble undifferentiated spermatogonia in G1 arrest from stage IV until stage VIII. Both populations of cells undergo the same morphological changes during this period, known to accompany transformation of most Aal spermatogonia into A1 spermatogonia in the adult. 3. Gonocytes in late G2 or prophase, 24 h p.p., resemble A1 spermatogonia in late G2 and prophase in stage IX. 4. Most of the daughter cells of the gonocytes resemble A2 spermatogonia, the others resembling adult undifferentiated spermatogonia. The hypothesis is put forward that gonocytes are identical to adult type undifferentiated spermatogonia. The primordial germ cells then give rise to adult type spermatogonial stem cells (As).