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]. [Illustration: 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. [Illustration: Fig. 160. Segmenting egg of _Trochus_. (After Robert.)] [Illustration: Fig. 161. Two views of segmenting egg of _Cynthia partita_. (After Conklin.)] [Illustration: Fig. 162. (_a_) Section of apical cone of _Salvinia_. (After Pringsheim[394].) (_b_) Diagram of probable actual arrangement.] [Illustration: Fig. 163. Egg of _Pyrosoma_. (After Korotneff).] [Illustration: Fig. 164. Egg of _Echinus_, segmenting under pressure. (After Driesch.)] In the accompanying series of figures (Figs. 160–167) I have {378} set forth a few aggregates of eight cells, mostly from drawings of segmenting eggs. In some cases they shew clearly the manner in which the cells meet one another, always at angles of 120°, and always with the help of five intermediate boundary walls within the eight-celled system; in other cases I have added a slightly altered drawing, so as to shew, with as little change as {379} possible, the arrangement of boundaries which probably actually existed, and gave rise to the appearance which the observer drew. These drawings may be compared with the various diagrams of Fig. 158, in which some seven out of the possible thirteen arrangements of five intermediate partitions (for a system of eight cells) have been already set forth. [Illustration: Fig. 165. (_a_) Part of segmenting egg of Cephalopod (after Watase); (_b_) probable actual arrangement.] [Illustration: Fig. 166. (_a_) Egg of _Echinus_; (_b_) do. of _Nereis_, under pressure. (After Driesch).] [Illustration: Fig. 167. (_a_) Egg of frog, under pressure (after Roux); (_b_) probable actual arrangement.]