Inside the green sphere are several smaller spheres.
In a still photograph, the arrangement suggests a simple story. Little versions wait inside a large one, grow, and release the still smaller versions hidden within them. Life resembles a set of nested toys, already complete and waiting to be opened.
Follow the development instead, and something happens that the photograph did not explain. At one stage, the young sphere is inside out. It must turn itself over.
Nobody does this with tweezers. Cells change shape, and their coordinated changes alter the configuration of the entire sheet. An opening widens, its edges turn outward, and what faced inward gradually comes to face out.
The next generation was not present all along as a finished miniature. This form has to develop again.
Different work within one sphere
The organism here is Volvox carteri, a widely studied species of green alga. There are other Volvox species, with differences in cell numbers, differentiation and inversion. One species' development should not be turned into a script for all of them.
In this species' asexual cycle, a mature sphere has many small somatic cells at its surface. Their flagella contribute to swimming. A smaller number of large reproductive cells lie toward the interior. Each can undergo successive divisions to produce an offspring.
The large cells are not aristocrats waiting to be served. Nor are the small ones workers on whom we can confer medals. They are different cells participating in the activities and development of one organism. Reading their positions as human social identities would take us away from what is happening.
The differences are nevertheless important. These are not simply many interchangeable cells gathered together. Their differentiation participates in how this sphere exists as a living organization.
The next generation does more than double the heap. A reproductive cell divides, differences arise among cells, and changes in form establish a new sphere with differentiated parts in different positions.
What continues includes the way those differences belong together.
Turning inside out
During development, the embryo's orientation is not yet the one required for the normally functioning adult. Inversion brings the prospective outer surface to the outside.
Look only at the completed organism, and the transition almost vanishes. This is simply what a Volvox looks like. Follow the earlier stages, and what appeared given turns out to have required a particular sequence of transformations.
We often picture growth as a change of size. A small tree becomes large; a child outgrows last year's clothes. Development also establishes differences and positions. It involves which parts form, where they belong and how activities become connected.
Inversion makes this unusually tangible. The embryo does not merely inflate an arrangement that was correct from the beginning. It passes through a stage that cannot be treated as a completed adult and changes its configuration through cellular activity.
Researchers can ask more precisely which cells change shape, how they are connected, and what disruptions prevent normal inversion. Such knowledge does not dismantle the scene until nothing worth looking at remains. It replaces a vague appeal to the miracle of life with more definite things to see.
The transformation has a sequence that can be reviewed and investigated. It does not need a narrator to announce that the organism has courageously turned itself over in order to survive.
Continuation is not the original escaping death
The developed offspring leave the parental sphere and begin their own lives. The parental somatic cells subsequently senesce and die.
Those events belong to the same life cycle. They do not add up to saying that the original did not really die. One living individual ends; others continue.
Nor does continuation establish that death was worthwhile, or that the remaining cells were obligated to exhaust themselves for descendants. Those are purposes and duties we would be inserting into the observation, then attributing to nature.
The beauty proposed here is not praise of death. A finite individual's kind of organization can arise again: not by dismantling the old body and transferring its assembled parts, but through development from a cell, establishing anew the differentiated activities that make the whole possible.
Beginning from one cell matters in this example. It is not a definition covering all reproduction. Cuttings, budding and other processes continue life in different ways. Nor must a new generation always involve sexual reproduction. The cycle described here does not require the union of two parents.
Stating the scope does not diminish the scene. We need not finish explaining the entire living world before attending carefully to this instance.
Difference does not explain itself
Individuals of the same species need not match exactly in cell number, size or developmental timing. A photocopy is therefore an inadequate image. That does not mean we have established the ultimate origin of every difference.
Nutrition, light, genetics and developmental processes can contribute. A cause not yet adequately measured is not automatically a cause that can never be explained.
The source paper advances a stronger reading: the differences retained in life's continuation include something external conditions cannot exhaustively account for. It sees here a becoming that is not merely the reprinting of an old form. But it also leaves a boundary explicit. Observed variation does not by itself prove this interpretation. The interpretation must remain answerable to empirical challenge.
An essay cannot discard that boundary for a more inspiring story. Otherwise it may leap from a developmental recording to the claim that every green sphere has a human kind of self. The image has not taken us across that distance.
We need not take the opposite shortcut either, supposing that anything explained by research becomes lifeless repetition. How an organism develops is one question. Our feelings about the explanation are another.
For now we can stay with the process: the return of a recognizable organization does not eliminate development and deliver a finished outcome directly.
The smaller spheres swim away
The busyness of inversion readily catches attention. Afterwards, when the new sphere's parts are in their places and it moves through the water, there is still something to see.
Formation has its tension; achieved form has its own completeness. Attention need not forever chase the next unfinished thing, as though life ceased to offer anything once a stage had been accomplished.
Which arrangement is most efficient, or which division of labor succeeds under particular conditions, can be investigated separately. Who should do what in a human life certainly cannot be decided by green spheres. A cell's developmental position is not a person's job from which departure must be forbidden. Beauty in reproduction creates no obligation for anyone to reproduce.
The recording can end without being made to teach us how to parent or govern.
At the end, the smaller spheres leave their former location. We can recognize what they are without claiming that the original has merely moved elsewhere.
The continuity is real. So is the new occurrence.