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.
Fig. 165. (a) Part of segmenting egg of Cephalopod (after Watase); (b) probable actual arrangement.
Fig. 166. (a) Egg of Echinus; (b) do. of Nereis, under pressure. (After Driesch).
Fig. 167. (a) Egg of frog, under pressure (after Roux); (b) probable actual arrangement.
It will be seen that M. Robert-Tornow’s figure of the segmenting egg of Trochus (Fig. [160]) clearly shews the cells grouped after the fashion of Fig. [158], a. In like manner, Mr Conklin’s figure of the ascidian egg (Cynthia) shews equally clearly the arrangement g.
A sea-urchin egg, segmenting under pressure, as figured by Driesch, scarcely requires any modification of the drawing to appear as a diagram of the type d. Turning for a moment to a botanical illustration, we have a figure of Pringsheim’s shewing an eight-celled stage in the apex of the young cone of Salvinia; it is in all probability referable, as in my modified diagram, to type c. Beside it is figured a very different object, a segmenting egg of the Ascidian Pyrosoma, after Korotneff; it may be that this also is to be referred to type c, but I think it is more easily referable to type b. For there is a difference between this diagram and that of Salvinia, in that here apparently, of the pairs of lateral cells, the upper and the lower cell are alternately the larger, while in the diagram of Salvinia the lower lateral cells both appear much larger than the upper ones; and this difference tallies with the appearance produced if we fill in the eight cells according to the type b or the type c. In the segmenting cuttlefish egg, there is again a slight dubiety as to which type it should be referred to, but it is in all probability referable, like Driesch’s Echinus egg, to d. Lastly, I have copied from Roux a curious figure of the egg of Rana esculenta, viewed from the animal pole, which appears to me referable, in all probability, to type g. Of type f, in which the five partitions form a figure with four re-entrant angles, that is to say a figure representing the five sides of a hexagon, I have found no examples among segmenting eggs, and that arrangement in all probability is a very unstable one.