The lid forming apparatus in cysts of the green algaAcetabularia mediterranea

Summary Cross sections and cross tangential sections of 1 to 3-day-old cysts (gametangia) ofAcetabularia mediterranea were examined by electron microscopy. In a defined zone of the peripheral cytoplasm of the cysts, where the lid is to be formed, a characteristic circular band-like structure, the putative lid forming apparatus, can be identified. In 1 -day-old cysts this structure is characterized by two electron dense amorphous layers close and parallel to the plasma membrane. In 3-day-old cysts the lower layer consists of rod-like structures. The position of the circular band-like lid forming apparatus is correlated to the position of the cyst organizing secondary nucleus which occupies a non central position. Usually the center of the lid forming apparatus lies on the shortest line between the secondary nucleus and the cyst wall. This suggests that the cyst organizing secondary nucleus plays an important role in the formation of the cyst lid.