During the present century oogenesis and maturation have been extensively studied in the Digenea. The earlier observations, however, were based on limited methods of histological and cytological techniques and were concerned principally with the general features of the germ-cells and with their chromosomes. Consequently, until recently, little information has emerged regarding the extra-nuclear components of oogonia and oocytes and the changes that these cells undergo during oogenesis. The earlier work on the Digenea is reviewed by Cable (1931), Anderson (1935), Rees (1939), and more recent research is cited in the present paper.

The work recorded in this paper was undertaken in order to obtain further knowledge regarding the behaviour of the cytoplasmic components of the male germ-cells of mammals and the contribution made by these bodies to the structure of the ripe sperm. Consequently, mammals from different groups were examined.Investigations were carried out independently by Zlotnik on the male germ-cells of the dog, the cat, and the rabbit, by Gresson on the boar and the ram, and jointly by Gresson and Zlotnik on the white rat and the golden hamster. Zlotnik (1943) originally identified and described the nuclear-ring of the spermatid and sperm of the dog. The accessory body was recognised simultaneously by him and by Gresson, but Zlotnik was the first to observe an accessory body within the localised Golgi material of the spermatocyte and to follow its extrusion to the cytoplasm, and, later, Gresson identified an accessory body within the Golgi material of the spermatid of the boar. In general the conclusions of the two authors are closely similar, such differences of detail or of interpretation as exist are discussed in this paper.

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While some variation of detail may be found in different species of the hermaphroditic Digenea, the process of spermatogenesis conforms to a common plan throughout the group. There is divergence of opinion concerning the interpretation of the structure of the ripe spermatozoon.In the testis of a young animal, primary, secondary and tertiary spermatogonia are present. The tertiary spermatogonia divide to give a group of eight primary spermatocytes that undergo the first division of the meiotic phase to form sixteen secondary spermatocytes. As a result of the second division of the meiotic phase, the sixteen secondary spermatocytes give a group of thirty-two spermatids that through a complicated series of changes are transformed into spermatozoa. During spermateleosis much of the cytoplasm of the late spermatid is sloughed off.Recent research, including the application of the Feulgen technique, the examination of living material, and electron microscopy, supports the view that the ripe spermatozoon is composed of an elongate head, and a flagellum. The head, with the exception of its external sheaths, consists of nuclear material.The flagellum of the sperm of Fasciola hepatica is Feulgen-negative and is composed of a proximal and a distal region. Its proximal part is provided with two axial filaments, a middle fibril and an external sheath. The axial filaments and the middle fibril are continued into the distal region where the external sheath appears to be absent. The external sheath of the flagellum of the sperm of Haematoloechus ( = Ostiolum) medioplexus is said to be composed of thirty-six fibrils. Claims that the flagellum of some digeneans contains a centriole, a middle-piece, and mitochondria have not so far been substantiated by means of electron microscopy.