Spermiogenesis of Nautilus pompilius. I. General survey

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Abstract
Differentiation of the spermatids, from secondary spermatocytes to mature spermatozoa in the spermatophores, is described for Nautilus pompilius. Early in development the spermatids have large cytoplasmic bridges which connect groups of cells which remain linked even when the residual cytoplasm is shed. The nucleus undergoes considerable elongation and the nuclear envelope loses its cisterna except in the centriolar fossa and adjacent to the acrosome. Condensation of the chromatin involves formation of dense sheets which run parallel to the long axis of the nucleus and eventually fuse to make a uniformly dense structure. The mitochondria migrate to one pole of the secondary spermatocyte where they apparently fuse to make two large mitochondria which elongate and eventually lie in grooves the whole length of the cylindrical nucleus. The centrioles are composed of nine sets of doublet microtubules and both migrate into the centriolar fossa. A massive manchette is formed which has extensive cross linked microtubules. The acrosome appears to arise from part of the Golgi apparatus and is a complex conical structure at the apex of the mature spermatozoan. The residual cytoplasm is shed by fusion of vesicles apparently derived from the nuclear envelope. Massive amounts of glycogen appear around the head in late spermiogenesis. Proximally, the flagellum has a sheath outside the microtubules which distally breaks up into nine coarse accessory fibers and further distally disappears. Nautilus sperm morphology is compared with the ultrastructure of other cephalopod spermatozoa and it is speculated that the differences found are greater than might be expected if the mechanisms of sperm transfer and fertilization were the only controlling forces determining the evolution of spermatozoan ultrastructure.