Ultrastructural changes in the germ plasm during the life cycle ofMiastor (Cecidomyidae, Diptera).

Polar granules are organelles unique to the germ plasm of some insects and amphibians and are thought to be involved in germ cell formation. These granules inMiastor are similar to those inDrosophila andAmphibia both in their structure and in their continuous presence in the germ line cells throughout the life cycle of the organism. They are formed during oogenesis as dense masses of amorphous material at the posterior tip of the oocyte. During the maturation and cleavage divisions of the embryo, the polar granules fragment into small granules and ribosomes become associated with their periphery. After their inclusion in the pole cell and the two pole cell divisions, the polar granules reaggregate into large granules and these subsequently become associated with the nuclear envelope as dense bodies. During the remainder of the life cycle ofMiastor until the inception of oogenesis, dense bodies are associated with the nuclear envelope of the primordial germ cells. During oogenesis the nuclear envelope of the oocyte lacks the dense bodies, but the nurse nuclei have copious amounts. The relationships between the dense bodies of the nuclear envelope of the nurse chamber and polar granules is discussed.