In _Phycomyces_ the organs of reproduction have been subjected to close examination by Van Tieghem,[C] and although he failed to discover chlamydospores in this, he describes them in other Mucors. In this species, besides the regular sexual development, by means of sporangia, there is a so-called sexual reproduction by means of zygospores, which takes place in this wise. The threads which conjugate to form the zygospores are slender and erect on the surface of the substratum. Two of these threads come into close contact through a considerable length, and clasp each other by alternate protuberances and depressions. Some of the protuberances are prolonged into slender tubes. At the same time the free extremities of the threads dilate, and arch over one towards the other until their tops touch like a vice, each limb of which rapidly increases in size. Each of these arcuate, clavate cells has now a portion of its extremity isolated by a partition, by means of which a new hemispherical cell is formed at the end of each thread at its point of junction with the opposed thread. These cells become afterwards cylindrical by pressure, the protoplasm is aggregated into a mass, the double membrane at the point of first contact is absorbed, and the two confluent masses of protoplasm form a zygospore invested with a tubercular coat and enveloped by the primary wall of the two conjugating cells. During this formation of the zygospore, the two arched cells whence the zygospore originated develop a series of dichotomous processes in close proximity to the walls which separate them from the zygospore. These processes appear at first on one of the arcuate cells in successive order. The first makes its appearance above upon the convex side; the succeeding ones to the right and left in descending order; the last is in the concavity beneath. It is only after the development of this that the first process appears on the opposite cell, which is followed by others in the same order. These dichotomous processes are nothing more than branches developed from the arcuate, or mother cells. During all these changes, while the zygospore enlarges, the wall of the arcuate cells becomes coloured brown. This colouring is more marked on the convex side, and it shows itself first in the cell on which the dichotomous branches are first produced, and which retains the darker tint longer than the other. The zone from whence the processes issue, and also the processes themselves, have their walls blackened deeply, while the walls of the conjugated cells, which continue to clothe the zygospore during the whole of its development, are bluish-black. By pressure, the thin brittle coat which envelopes the zygospore is ruptured, and the coat of the zygospore exposed, formed of a thick cartilaginous membrane, studded with large irregular warts.