The differentiation of the spermatids of Bufo arenarum has been described from a study of electron micrographs of thin sections of testis.

The development of the acrosome from the Golgi complex takes place in much the same manner as in mammalian spermatogenesis but no acrosome granule is formed.

A perforatorium is described for the first time in this species. It is formed by a convergence of dense filaments that arise between the nuclear membrane and the head cap.

During maturation of the spermatid the chromatin undergoes striking physicochemical alterations. Fine chromatin granules uniformly dispersed in the karyoplasm are replaced by larger and larger aggregates and these ultimately coalesce to form a very dense sperm head.

Two centrioles of cylindrical form are situated very near the base of the sperm head. The longitudinal fibrils of the tail flagellum take origin from one, and the dense fibrous substance of the undulating membrane is closely related to the other.

Phase contrast cinematographic observations on the swimming movements of living toad sperm, when considered in relation to the fine structural components of the tail, suggest that there is a contractile component in the undulating membrane as well as in the axial fibrils.

The differences in the structure of mammalian and amphibian sperm tails are discussed in relation to differences in the character of their movements.

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