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SUMMARYAs the zygospore of Rhizopus sexualis enlarges it develops a thick wall. Studies with the transmission and stereoscan electron microscopes show that this wall is formed in the following manner. (I) The gametangial septa increase in thickness by the deposition of new material, which is laid down more rapidly on the inner or zygospore side than on the suspensor side. These septa form the 'end walls' of the zygospore. Fine plasmodesmatal tubules remain at intervals in the wall and are in direct connexion with the tubules of a dense zone of endoplasmic reticulum situated on either side of the septa. It is considered likely that the plasmodesmata allow the passage of nutrients from the parent hyphae into the developing zygospore. (2) The 'side wall' of the zygospore is formed by the development of new patches within the original wall of the fused gametangia. At first these 'patches' are shaped like inverted saucers and are separate from each other, except that there is a more continuous zone adjacent to the end walls of the spore. Then, the 'patches' become larger, by the deposition of new material at the rim, finally becoming shaped like inverted flower pots and coalescing at the rims to give a continuous warted layer. Until this stage the zygospore wall is permeable to water and dissolved substances. Meanwhile the original wall h s t becomes gelatinous, then membranous, and finally tears and is sloughed off exposing the newly-formed sculptured wall. (3) Finally a new inner wall, impermeable to water and dissolved substances, is laid down within the ornamented wall.

The development of the septa which cut off the gametangia of Rhizopus sexualis from the suspensor cells was studied by electron microscopy. The edge of the ingrowing septum extends by the incorporation of coalesced vesicles which were previously aligned in the plane of the developing septum. The membranes surrounding these vesicles fuse to form the plasmalemmas of the gametangium and suspensor cell and are eventually continuous with the plasmalemma of the original progametangium. New wall material is laid down between these plasmalemmas. Associated with this process is a considerable increase in complexity of the endoplasmic reticulum which forms broad, irregular bands on either side of the septum. Fine tubules, resembling plasmodesmata, pass through the wall at intervals. Thickening of the septum proceeds more rapidly on the gametangial side. I N T R O D U C T I O NSoon after a pair of progametangia of Rhizopus sexualis make contact at their apices, the terminal parts (gametangia) become cut off from the suspensor cells by the formation of transverse septa. This paper describes the process of formation of these septa as revealed by the electron microscope. METHODSThe fungus was grown on cellophane laid on the surface of malt-agar plates. Young colonies with all stages of zygospore development present were fixed in 2 yo (w/v unbuffered KMn04 for 30 min. at room temperature, and washed several times in distilled water. The hyphae became brittle during fixation and the zygospores were readily detached from the mycelium during washing. Zygospores were then collected in centrifuge tubes, dehydrated, and embedded in Epon or Vestopal. Sections were cut at approximately 450-550A thickness. RESULTSThe first sign of septum development detectable by light microscopy is a slight decrease in density of the cell contents on the inner side of the enveloping walls of the progametangium at the site from which the septum would be expected to develop. The electron microscope revealed, however, that in such a specimen a thin wall (about 135oA thick at the junction with the progametangial wall and tapering to about 385A thickness at the advancing inner edge) was already parti ally developed, extending inwards from the peripheral wall and leaving a central gap of about one third of the

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SUMMARYConjugating progametangia of Rhizopus sexualis rapidly enlarge and become flattened at the apical zone of contact. The walls over this zone fuse immediately, lose their separate identity and form a single plate. The fusion wall then thickens by gelatinization and swelling: there is no evidence of deposition of secondary wall material. The plasmalemmas on either side of the fusion wall become wrinkled and enclose pockets of electron transparent material along the sides adjacent to the wall. Vesicles and later lomasome-like masses are seen in a few of these pockets. Numerous small vesicles are present in the adjacent zones of cytoplasm on either side of the wall; some may be seen attached to the plasmalemmas. The wall then thins from the centre of the plate outwards but remains in being until after the gametangia have been delimited by new septa. Cell fusion then takes place by the development at the centre of the fusion wall of a hole which enlarges rapidly until the whole of the plate is dissolved. The evidence from electron micrographs suggests that the dissolution process is entirely chemical and that both matrix and microfibrillar skeleton are dissolved enzymatically while still enclosed in the plasmalemma.