THE ULTRASTRUCTURE OF SPERMATOGENESIS AND EPIDIDYMAL SPERMATOZOA OF THE TUATARA SPHENODON-PUNCTATUS (SPHENODONTIDA, AMNIOTA)

By using transmission electron microscopy (TEM) the events of spermato genesis are described for the first time in the tuatara Sphenodon punc tatus punctatus (Gray), a representative of the 'reptilian' order Sphe nodontida. Secondary spermatocytes contain two greatly elongate (8.0 m u m), rod-shaped centrioles which lie parallel to one another and are each associated with a small deposit of dense material and a short cen triole. Spermatids contain only one rod-shaped centriole (associated w ith a short centriole) which Fives rise to the flagellar axoneme there by becoming the distal centriole. Four stages of spermatid development can be distinguished: (i) the early stage (nucleus round; nuclear con tents granular with a thin, condensed periphery; mitochondria scattere d; acrosomal vesicle spheroidal, slightly depressed onto nuclear surfa ce); (ii) the middle stage (nucleus pyriform with two endonuclear cana ls formed; nuclear contents fibro-granular with thick periphery; mitoc hondria chiefly posterior; acrosomal vesicle flattened; centriolar com plex attached to nucleus); (iii) the advanced stage (nucleus elongate and rod shaped; nuclear contents coarsely granular; mitochondria (cont aining linear cristae) clustered around the distal centriole; acrosoma l vesicle conical; centriolar complex attached to posterior fossa of n ucleus); (iv) the late stage (nucleus very elongate and associated wit h a longitudinally arranged microtubular sheath; nuclear contents very condensed; midpiece fully formed and featuring mitochondria with conc entric cristae and a dense intramitochondrial body; centrioles associa ted with a dense, lateral body). Testicular sperm have a conical acros omal vesicle (length 4 mu m) and subacrosomal cone, an elongate (lengt h 54-56 mu m) helical nucleus, a midpiece (length 8 mu m, featuring sp heroidal mitochondria containing concentric cristae and a dense body), an annulus, an elongate principal piece (length 74-78 mu m, featuring a dense, fibrous sheath) and a short end piece (length 2-4 mu m). Epi didymal sperm differ from those in the testis by having a more develop ed lateral body in the midpiece and a sheath of flocculent material su rrounding the fibrous sheath in the principal piece. The relatively la rge number of epididymal sperm still associated with a cytoplasmic dro plet suggests that sperm spend a significant period maturing within th e epididymis. The features of spermatogenesis and mature sperm suggest that the Sphenodontida are primitive amniotes, with only chelonians h aving fewer spermatozoal apomorphies while the crocodilians are little more advanced.