Fine Structure of the Reproductive System of a Frog Lung Fluke. II. Penetration of the Ovum by a Spermatozoon

In the trematode, Haematoloechus medioplexus, the plasma membrane of the spermatozoon fuses, along the entire length of the cell, with that of the ovum. An acrosome is not present in these spermatozoa. Upon fusion of gametes, the plasma membrane and peripheral microtubules of the spermatozoon remain associated with the surface of the ovum, although the surface area of the ovum is not increased in proportion to that of the spermatozoon with which it unites. After fusion, however, the volume of the ovum is increased by about 200 /3, which is the approximate volume of an entire spermatozoon. In connection with a recent study (Burton, 1967) of Mehlis' gland of a frog lung fluke (Haematoloechus medioplexus), it was noted that before the eggshell becomes sclerotized, it is possible to obtain ultrathin sections which show stages in the penetration of an ovum by a filamentous spermatozoon. The present electron microscopic observations clarify and extend knowledge of some of the early events in fertilization in this trematode, as described in a previous article based on studies with the light microscope (Burton, 1960). For brevity, the term ovum is used in this paper for the female cell after it leaves the ovary, although it is a primary oocyte until meiosis is completed following penetration by the spermato-