Examples of these various arrangements meet us at every turn, and not only in cell-aggregates, but in various cases where non-rigid and semi-fluid partitions (or partitions that were so to begin with) meet together. And it is a necessary consequence of this physical phenomenon, and of the limited and very small number of possible arrangements, that we get similar appearances, capable of representation by the same diagram, in the most diverse fields of biology[393].
Fig. 159.
Among the published figures of embryonic stages and other cell aggregates, we only discern these little intermediate partitions in cases where the investigator has drawn carefully just what lay before him, without any preconceived notions as to radial or other symmetry; but even in other cases we can generally recognise, without much difficulty, what the actual arrangement was whereby the cell-walls met together in equilibrium. I have a strong suspicion that a leaning towards Sachs’s Rule, that one cell-wall tends to set itself at right angles to another cell-wall (a rule whose strict limitations, and narrow range of application, we have already {377} considered) is responsible for many inaccurate or incomplete representations of the mutual arrangement of aggregated cells.
| Fig. 160. Segmenting egg of Trochus. (After Robert.) | Fig. 161. Two views of segmenting egg of Cynthia partita. (After Conklin.) |
Fig. 162. (a) Section of apical cone of Salvinia. (After Pringsheim[394].) (b) Diagram of probable actual arrangement.
| Fig. 163. Egg of Pyrosoma. (After Korotneff). | Fig. 164. Egg of Echinus, segmenting under pressure. (After Driesch.) |