Microtubules in the spermatids of stick insects.

Spermatids from two phasmid species were seen to possess an unusually large amount of microtubules along the nucleus and tail. Some of the microtubules have a loosely fitting sleeve for half a micron or more. During late stages in spermiogenesis the microtubules aggregate and form one or several "microtubular crystals" consisting of electron-lucid tubular elements with a diameter of about 360 A. The tail flagellum contains five kinds of microtubular structures, which all have a substructure of longitudinal protofilaments that is clearly visible after fixation in the presence of tannic acid. The so-called accessory tubules have 17 protofilaments that have the same appearance as that in ordinary, 13-unit microtubules, but are somewhat thicker than those. It is evident that the protofilaments in both the 17-unit and the 13-unit microtubules run parallel or nearly parallel to the long axis of the microtubules. It is of interest that both types of microtubules possess a prime number of protofilaments which may give the fagellum certain functional advantages.