This is an observation from Raymond Goldstein's group in the Department of Applied Mathematics and Theoretical Physics at Cambridge. The first author is Stephanie Höhn, with Aurelia Honerkamp-Smith, Pierre Haas and Philipp Khuc Trong, published 27 April 2015 in Physical Review Letters, volume 114, article 178101. The subject is the embryo of Volvox: a hollow ball of cells that has to turn itself completely inside out. Using light-sheet microscopy, they recorded the first live three-dimensional images of the process. The inversion takes about an hour: the sphere folds inward around its middle into two hemispheres, one hemisphere moves inside the other, an opening at the top widens, and the outer hemisphere glides over the inner one until the sphere is restored.

They did not stop at filming. The paper opens by saying that spherical embryos turn themselves inside out through a process hypothesized to arise from cell shape changes alone. They modelled those shape changes as changes of intrinsic curvature in an elastic shell, and asked: is that enough? The answer: one hemisphere must actively contract and the other actively expand, in a coordinated pattern in time and space. In the Cambridge press release Goldstein says that until then there was no quantitative mechanical understanding of whether those cell shape changes were sufficient.

This is the generative face of the seventh ray, the Beauty of Reproduction, and nearly the ray's own example: a small sphere turning itself inside out. The ray's sentence runs: its parts differ, each in its own place, and only together are it; it dies, but its pattern does not die with it, and the whole of it grows again from a single cell. A mature Volvox has about two thousand small flagellated cells that row, and about a dozen large germ cells. The rowers die together, but each germ cell has already divided into a complete new sphere, which inverts and leaves the mother. The inversion is exactly the step of growing again: a thin sheet of cells with no outside mould, whose regions take up different roles (this side contracts, that side stretches), so that the whole shape is turned over. What a judge cannot not acknowledge here is that this was not pressed into shape from outside, and is not one cell simply splitting in two: difference became division of labour, and division of labour became a form. Nor is it merely useful or clever. If no one cared about Volvox, a thin shell turning itself over through the differing actions of its own parts would still stand.

It is worth saying what this is not. It is not praise of vital force, nor of how cleverly evolution designed it. The strength of the paper is that it brings the event into mechanics that can be tested, without letting the mechanics swallow the event. Mechanism and beauty are not the same dimension here; the clearer the first becomes, the less the second is diminished.

The remainder is also stated in the paper's own terms. The model shows that cell shape changes can suffice, but how the active contraction and expansion are coordinated in the living embryo, and whether the hemisphere changes come from forces inside the invaginating region or from elsewhere, is still open; the authors also suggest the approach may extend to processes such as neurulation in animals. A second remainder belongs to the ray itself: what grows again each generation is not entirely the same. That hour of inversion is the hour in which the pattern starts from nothing once more, and is not fully fixed by what is outside it.

It is worth seeing now because it can be pointed to directly: a living event of one hour, a ball with no hands and no mould that turns itself over. After seeing it, it is hard to treat shape as only a result. The original paper:

Han Qin (秦汉) · @hqinjarsy · SAE Judgment & Aesthetics · Ray 7