Fertilization in a crab: I. Early events in the ovary, and cytological aspects of the acrosome reaction and gamete contacts.

Early events of fertilization were studied in Carcinus maenas by in vitro experiments and ultrastructural analysis; some were found to occur in the lumen of ripe ovaries. The acrosome reaction generally conformed to the usual. Reptantia Decapoda pattern. However, a prominent membrane system continuous with the nuclear envelope and located close to the base of the acrosome tubule characterized the type of spermatozoon observed in Carcinus maenas. Such complex anatomical connections linking the three parts of the reacted spermatozoon (acrosome tubule, membrane system and nucleus envelope) may be significant in relation to the membrane system's contribution to the acrosome reaction. The outer layer of the everted acrosomal vesicle was found to comprise tubular elements ending in bell-shaped corpuscles, deeply interdigitated with the oolemma microvilli during the establishment of the initial contacts between the reacted spermatozoon and the egg plasma membrane. At the site of contact, the oolemma formed in minute fertilization cone, locally depressed by the acrosome tubule. During these early fertilization events, the nucleus, like the other spermatozoon components, was seen to penetrate the egg coatings first, the later to be located near the oolemma.