of a system of hexagons, or a system of dodecahedra.
Histologically, the case is illustrated by a well-known phenomenon in embryology. In the segmenting ovum, there is a tendency for the cells to be budded off in linear series; and so they often remain, in rows side by side, at least for a considerable time and during the course of several consecutive cell divisions. Such an arrangement constitutes what the embryologists call the ãradial typeã of segmentation57. But in what is described as the ãspiral typeã of segmentation, it is stated that, as soon as the first horizontal furrow has divided the cells into an upper and a lower layer, those of ãthe upper layer are shifted in respect to the lower layer, by means of a rotation about the vertical axis58.ã It is, of course, evident that the whole process is merely that which is familiar to physicists as ãclose packing.ã It is a very simple case of what Lord Kelvin used to call ãa problem in tactics.ã It is a mere question of the rigidity of the system, of the freedom of movement on the part of its constituent cells, whether or at what stage this tendency to slip into the closest propinquity, or position of minimum potential, will be found to manifest itself.
However the hexactinellid spicules be arranged (and this is {454} not at all easy to determine) in relation to the tissues and chambers of the sponge, it is at least clear that, whether they be separate or be fused together (as often happens) in a composite skeleton, they effect a symmetrical partitioning of space according to the cubical system, in contrast to that closer packing which is represented and effected by the tetrahedral system59.
This question of the origin and causation of the forms of sponge-spicules, with which we have now briefly dealt, is all the more important and all the more interesting because it has been discussed time and again, from points of view which are charôÙacôÙterôÙisôÙtic of very different schools of thought in biology. Haeckel found in the form of the sponge-spicule a typical illustration of his theory of ãbio-cryôÙstalôÙliôÙsaôÙtionã; he considered that these ãbiocrystalsã represented ãsomething midwayãein Mitteldingãbetween an inorganic crystal and an organic secretionã; that there was a ãcompromise between the crystallising efforts of the calcium carbonate and the formative activity of the fused cells of the syncytiumã; and that the semi-crystalline secretions of calcium carbonate ãwere utilised by natural selection as ãspiculesã for building up a skeleton, and