Orphan Unsolved Problems

A tennis player's serving arm muscles and arm bones become much stronger than the other arm. (And the trabeculi of bone marrow get preferentially aligned parallel and perpendicular to maximum force).

Astronauts' bones and muscles weaken within days.

Either osteoclasts respond to increased stress by dissolving less bone;
OR osteocytes respond to load by depositing more bone.
Conversely, non-stressed bones either inhibits bone deposition or stimulates osteoclasts

It is also not known whether response is the maximum force, the average force, the frequency of forces. Athletic coaches presumably acquire intuitive understanding of how to optimize such stimuli.

Epidermis of your hands thickens where rubbed the most; Calluses form and thicken. "Chalone theory" that epidermal cells secrete a self-inhibitory chemical, that leaks away wherever stress is strongest.
 

II) Spontaneous Perpendicularity of Collagen Fibers and Muscles.
Myoepithelial cells of Hydra
Skeletal muscle cells of mammal tongues
Collagen fiber layers in vertebrate corneas
Notochord sheath: Alternating collagen layers that spiral around the vacuolated cells
 

III) Self-Repositioning of embryonic tissuesor organs in response to surgical rotation and displacement.
Nardi and Stocum, etc., Michael Locke.
 

IV) Polyembryony Some parasitic wasps develop from random piles of undifferentiated cells, instead of from embryos. Adult wasps (of just certain species) inject fertilized egg cells unto caterpillar body cavities. The undifferentiated wasp cells grow and divide mitotically, forming random piles of thousands of cells.

Clusters of these undifferentiated wasp cells eventually aggregate together, and develop into wasp larvae.
Apparently they skip gastrulation, neurulation, segment formation.
But not nearly enough research has yet been done on this alternate type of development. Sacculina is a genus of parasitic barnacle that preys on Blue Claw Crabs.
Tiny individuals attach to the crab, and extends long branching cytoplasmic strands all through the crab body, resembling mycelia of fungi. Large masses of barnacle eggs then form on the crabs lower surface, at the place where crab eggs would normally have developed, Each parasite starts as a typical swimming arthropod body. If these attacked humans, they would be a good subject for a horror movie.
https://en.wikipedia.org/wiki/Sacculina

V) Some invertebrates (colonial sea squirts) can reproduce both by eggs, that gastrulate, neurulate etc. or alternatively by asexual budding, with different sequences of cell rearrangements (differing from gastrulation). In the one species that I studied, pigment cells detached into the blood and colonized asexual buds. Botryllus schlosseri. This species lives on leaves of kelp from Long Island northward.
 

VI) Tension-Dependent Embryonic Regularity Bird feathers and scales of snakes and lizards develop from very regularly-spaced clumps of dermal fibroblasts. This regularity depends on the dermis being stretched tight. If the dermis is limp, then feathers and scales develop irregular spacing. This is an extreme example of a phenomenon with many other examples of spatial regularity depending on tension. The explanation, function or cause is not known. Textbooks don't mention the phenomenon. There is no agreed-upon name for this phenomenon.