. Electron microscopy; proceedings of the Stockholm Conference, September, 1956 . Fig. 2. Detail of a head with part of the "caudal sheath" and the nuclear material. Magnification 103,000. acrosomal region consists of at least four different parts, the mutual relations of which are not entirely clear in this material. The acrosomal barbs have a homogeneous texture. The nucleus, as seen in fig. 1., has a strikingly regular ultrastructure. There are many uniformly thick opaque filaments in a parallel arrangement oriented along the long axis of the nucleus. In a transverse section at th
. Electron microscopy; proceedings of the Stockholm Conference, September, 1956 . Fig. 2. Detail of a head with part of the "caudal sheath" and the nuclear material. Magnification 103,000. acrosomal region consists of at least four different parts, the mutual relations of which are not entirely clear in this material. The acrosomal barbs have a homogeneous texture. The nucleus, as seen in fig. 1., has a strikingly regular ultrastructure. There are many uniformly thick opaque filaments in a parallel arrangement oriented along the long axis of the nucleus. In a transverse section at the broadest part of the nucleus approximately 3000 filaments, about 200 A in dia- meter, have been counted. The filaments are within restricted regions equally spaced in a hexagonal array giving a crystal-like appearance to that region of the nucleus. In longitudinal sections these filaments could be followed for a distance of several microns. There is no clear evidence that two adjacent filaments join at their ends. Towards the basal portion of the nucleus there is a general coarsening of the structure. Instead of the filaments dense conglomerates of irregularly sized opaque regions are observed near the centriole. The nuclear membrane shows a varying appearance in different regions. Partially it seems to consist of a single layer and partially a triple layered structure is evident. In the neck region and alongside the convex surface of the head of the spermatid a caudal sheath is found, resembling very closely the caudal sheath described in the cat spermatid by Burgos and Faw- cett (2), or that in the spermatozoon of the edible snail (6). This sheath is thus seen to consist of a cylindrical aggregation of apparently tubular fila- ments. Possibly the function of the caudal sheath is a supporting one as it covers the neck region, where the mechanical strains are at their greatest. The centriole exhibits no peculiarities and is not found in a nuclear indentation as is the centrio
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