Testes of Eriocheir japonicus were fixed in buffered (pH 7.2) 1 per cent osmium tetroxide and thin sections of the methacrylate-embedded tissue were studied with the electron microscope. Spermatozoa from the vasa deferentia and spermatids from the testes were examined in smear preparations and sections. The most useful stainings proved to be the Feulgen, Unna-Pappenheim, and PAS reactions.

The present paper covers one of the late stages of spermatogenesis. At the late stage of differentiation of the spermatid, the nucleus shows a concave disc-like contour in longitudinal sections and the karyoplasm is finely alveolar in appearance. A vesicle with a fibrillar or granular content appears situated between the cytoplasm and the nucleus. In an advanced spermatid, a large vacuole develops between the nucleus and this vesicle. As development of the spermatid proceeds, the cytoplasm is sloughed off. Thus, the advanced spermatid consists of an ovoid vesicle surrounded by a flattened nuclear disc. The former corresponds to the "capsule" or the "head," and the latter to the "pseudopodia." Concurrently with the sloughing off of the cytoplasm, a lamellar structure appears at the periphery of the head. It is composed of thin plates imbricated like the leaves of an onion. Each plate consists of triple layers, two dense layers separated by a lighter space. Each layer measures about 7 mµ in diameter.

At the distal pole of the head, a depression develops into a tubule which reaches the proximal pole of this structure. The lumen of the tubule is occupied by a dense substance part of which arises from the limiting membrane of the vacuole. The dense material begins to fill the tubule from the distal towards the proximal end. It finds its way till the middle of the tubule, leaving its proximal half unfiled. Thus, the proximal portion of the mature sperm head has a straight tubule whlich opens on its surface. The latter is covered by a dense membrane about 60 mµ in width.

Cytochemical analysis reveals that the pseudopodia contain DNA, and the head a carbohydrate component as well as PNA. The classical concept that the head becomes the male pronucleus after fertilization can no longer be accepted on the basis of the cytochemical analysis. The present study came to no definite conclusions concerning the functional significance of the head.

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