The Driesch Phenomenon: Long a Holy Grail of Developmental Biology.
Adjustments of Anatomical Dimensions; Scaling; "Regulative Embryos"

Different-sized Dictyostelium slugs maintain about the same ratios of length to width, and the same proportionality of stalk height/spore mass volumes. Big slugs form taller fruiting bodies, etc. By what mechanism?(s)?

As Hans Driesch discovered (1890s), separating the first 4 cells of starfish (or other echinoderm) cleavage-stage embryos results in 1/4 sized larvae. Sticking two one-cell stages together results in a double-sized larva.
But W. Roux discovered that killing one of the first 2 cells, does NOT result in the undamaged cell "regulating" in the sense of forming a half-sized larva!

By what mechanism(s?) do early stages of sea urchin embryos adjust the sizes of their organs in proportion? Biologists have never gotten their minds around these questions. Can Molecular Biology get closer to answers?

Please try to invent possible causes; And experiments to test them.
Driesch's explanation was tiny mind-like Aristotelian Enteleches.
Lewis Wolpert invented "Positional Information" specifically to explain this!
They both seem about equally meaningless to me; But what do you think?

Many biologists confuse "positional information" with "prepatterns."
Discovery that signaling molecules like wnt are very similar in flies & people has been interpreted as confirming predictions of "positional information" Hox genes even more so! What do you think?

Don't bother to read this book about "purposefulness of developing embryos.
The Wikipedia article about Driesch credits him with the first cloning.
As Wolfgang Pauli said about a vacuous lecture: "It's not even wrong"

But philosophizing need not always be empty. Embryology's problem of shape "regulation" is a mega-problem, or meta-problem, comparable to "How can nerve networks learn, remember or think", "How does music work", etc.