similar to that observed
in other species of _Mucor_. In these the rough tuberculate epispore
splits on one side, and its internal coat elongates itself and
protrudes as a tube filled with protoplasm and oil globules,
terminating in an ordinary sporangium. Usually the amount of nutriment
contained in the zygospore is exhausted by the formation of the
terminal sporangium, according to Brefeld;[D] but Van Tieghem and Le
Monnier remark that in their examinations they have often seen a
partition formed at about a third of the length of the principal
filament from the base, below which a strong branch is given off, and
this is also terminated by a large sporangium.

[Illustration: FIG. 96.--Zygospore of _Rhizopus_ in different stages. (De
Bary.)]

De Bary has given a precise account of the formation of the zygospore
in another of the Mucors, _Rhizopus nigricans_, in which he says that
the filaments which conjugate are solid rampant tubes, which are
branched without order and confusedly intermingled. Where two of these
filaments meet each of them pushes towards the other an appendage
which is at first cylindrical and of the same diameter. From the first
these two processes are applied firmly one to the other by their
extremities; they increase in size, become clavate, and constitute
together a fusiform body placed across the two conjugated filaments.
Between the two halves of this body there exists no constant
difference of size; often they are both perfectly equal. In each
there is collected an abundance of protoplasm, and when they have
attained a certain development the largest extremity of each is
isolated by a septum from the clavule, which thus becomes the support
or suspender of the copulative cell. The two conjugated cells of the
fusiform body are generally unequal; the one is a cylinder as long as
it is broad, the other is disciform, and its length is only equal to
half its breadth. The primitive membrane of the clavule forms between
the copulative cells a solid partition of two membranes, but soon
after the cells have become defined the medial partition becomes
pierced in the centre, and then soon entirely disappears, so that the
two twin cells are confounded in one single zygospore, which is due to
the union of two more or less similar utricles. After its formation
the zygospore still increases considerably in size, and acquires a
diameter of more than one-fifth of a millimetre. Its form is generally
spherical,