IX] OF HEXACTINELLID SPICULES 689 



throwing the difficulty a Uttle further back, we may so far account 

 for them by considering that the cells or vesicles by which they 

 are conformed are not arranged in what is known as '/closest 

 packing," but in hnear series; so that in their arrangement, and 

 by their mutual compression, we tend to get a pattern not of 

 hexagons but of squares: or, looking to the solid, not of dodeca- 

 hedra but of cubes or parallelepipeda. This indeed appears to be 

 the case, not with the individual cells (in the histological sense), 

 but with the larger units or vesicles, which make up the body of the 

 hexactinelHd. And this being so, the spicules formed between the 

 linear, or cubical series of vesicles, will have the same tendency 

 towards a " hexactinelhd " shape, corresponding to the angles and 

 adjacent edges of a system of cubes, as in our former case they had 

 to a triradiate or a te tract inellid form, when developed in connection 

 with the angles and edges of a system of hexagons, or a system of 

 rhombic dodecahedra. 



However the hexactinelhd spicules be arranged (and this is none 

 too easy to determine) in relation to the tissues and chambers of 

 the sponge, it is at least clear that, whether they lie separate or 

 be fused together in a composite skeleton, th-ey 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 system^*. 



Histologically, the case is illustrated by a. well-known pheno- 

 menon 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 "radial 

 type" of segmentationf. But in what is described as the "spiral 

 type" of segmentation, it is stated that, as soon as the first hori- 

 zontal furrow has divided the cells into an upper and a lower layer, 

 those of "the upper layer are shifted in respect to the lower layer, 



* Chall. Hep., Hexactinellida, pis. xvi. liii, Ixxvi, Ixxxviii. 



t See, for instance, the figures of Ihe segmenting egg of Synapta (after Selenka), 

 in Korschelt and Heider's Vergleichende Entvnckinngsgeschichte. On the spiral 

 type of segmentation as a secondary derivative, due to mechanical causes, of the 

 "radial" type of segmentation, see E. B. Wilson, Cell-lineage of Nereis, Journ, 

 Morph. VI, p. 450, 1892. 



