Scanning electron microscopy (SEM) image of testis cells from a newborn piglet cultured for four weeks. Imaged for the first time, this is a large colony formed by fusion of early testis germ cells named gonocytes. Gonocytes are progenitors of all generations of testis germ cells. Also shown here are multiple migrating gonocytes approaching the colony to form an even bigger colony. The process of gonocyte migration and multiplication is crucial for the normal initiation of sperm production later in a male's life. The single gonocytes and the large colony are surrounded by flat supporting somatic testis cells. (Magnification 1000x).