SUMMARY: As the zygospore of enlarges it develops a thick wall. Studies with the transmission and stereoscan electron microscopes show that this wall is formed in the following manner. (1) 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 first 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.


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