Occasionally one of the tetrads takes the form of a cross (fig. 249).
In one stage these double U's look much like tetrads (fig. 138).
A whole Irish law-book is composed in the form of heptads; while triads, tetrads, &c., occur in every part of the Laws.
If there are no sarcina, but tetrads or staphylococci (rarely merismopedia), then the division is in two planes.
When such rings do occur their genesis is very obscure, and at no time do they present the appearance of “tetrads.”
In each sporangium large numbers of these tetrads develop and all the ripe spores seem to have been of one size.
The members of the group are all tetrads, and have eight funnels, opening on the eight faces of the octahedron.