THE ULTRASTRUCTURE OF THE MATURE OOCYTE AND THE NURSE CELLS OF THE CERACTINOMORPHA PETROSIA-FICIFORMIS

Petrosia ficiformis (Poiret), one of the Demospongiae Ceractinomorpha, is an oviparous species having relatively large oocytes (140-160 mu m ) and a nucleolated nucleus. The perinuclear region is devoid of yolk inclusions and contains an elevated number of dictyosomes and ergastop lasmic lamellae as well as numerous mitochondrial clusters. In compari son the other parts of the cytoplasm are rich in variously organized i nclusions. The oocyte is surrounded by archeocyte-type nurse cells. Ce ll contacts via pseudopods on both cell surfaces, and spherical electr on-dense corpuscles in spaces between nurse cells and oocyte have been seen. It is suggested that in Petrosia ficiformis, because of the pre sence of the nurse cells, the majority of the yolk material is derived from these cells and is then subsequently elaborated. A part of the y olk is also formed through the autosynthetic activity of the oocyte it self.