Ultrastructure of the embryonic envelopes of Proteocephalus filicollis Rudolphi (Cestode: Proteocephalidea).
A study of the ultrastructure of the embryonic envelopes of P. filicollis obtained from freshly-killed three-spined stickleback (Gasterosteus aculeatus) in Scotland demonstrated that there are four embryonic envelopes around the mature egg. These are the capsule, the outer envelope, the inner envelope and the oncospheral membrane. The capsule is the outermost envelope which surrounds the embryo and is composed of two apposed membranes. The outer envelope probably originates from two macromers. Cellular organelles including mitochondria, cisternae of endoplasmic reticulum, polysomes, vesicles of different sizes and lipid droplets are present in the outer envelope which is filled with electron-dense granular material. The inner envelopes appear at the formation of the preoncosphere and contain mitochondria and vesicles. These vesicles fuse to eventually form larger intercellular spaces. Two distinct parts appear in the inner envelope. The oncospheral membrane is formed at the end of embryogenesis and appears to rise by delamination from the outer surface of the oncosphere.