Spermatozoan vesicle loss during penetration of the female gonangium in the hydroid. Campanularia flexuosa

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Abstract
Fertilization in the hydrozoan Campanularia flexuosa is internal and may be considered to include events in the female reproductive organ (the gonangium) prior to zygote formation. Prior to fertilization the spermatozoan must move through the female gonangium to the site of sperm‐egg fusion. The female gonangium contains sheets of epithelial cells which distally line the entrance and proximally form passageways that lead to and surround the eggs. After chemotactic attraction to the gonangium aperture, the spermatozoa adhere to the epithelial cells by their outer surfaces, but they continue to move downward along the distal gonangium epithelia and through the passageways to the germinal vesicle area of the egg. As the spermatozoa move deeper into the gonangium the 640–700 Å membrane‐bounded (“pro‐acrosomal”) vesicles of the sperm progressively decrease in number. By the time the spermatozoa reach the egg, 88% of the vesicles have been lost. It is possible that the contents of the vesicles play a role in the control of spermatozoan‐epithelial cell adhesion or that they serve, in some way, to permit sperm penetration to the egg surface. Alternatively, their loss may be a morphological indication of the capacitation‐like interaction between the sperm and normal female gonangium epithelial cells which is required for fertilization in Campanularia flexuosa.