Embryo Geometry Revisited
Morphogenesis and Embryogenesis:
- Blastula bursts and splits along midline
- Surface [tissue] of blastula undergoes elastic recoil
- Components of embryonic body are compressed
- Embryonic components (including limb buds and somites), free of constraint, expand and recognizable [species specific] fetal features appear
The vertebrate embryo is a bizarre shape that seems to emerge de novo from a small mass of cells that gathers inside the ball of cells resulting from the subdivision of the fertilized egg.
One of the most perplexing aspects of development is the remarkable similarity of embryonic form across all the vertebrate groups—from fish to human. Vertebrate embryos seem almost identical to one another, yet they bear little resemblance to subsequent juvenile and adult forms.
To address this conundrum, we suggest that the morphology of the vertebrate embryo may represent a transient stage of development. During this stage, we propose that the bursting and splitting of the blastula along its midline induces the elastic recoil of its surface, yielding a compressed form of the developing body. We propose these mechanical events as the key features of gastrulation: a mysterious phase of development until now.
Following gastrulation, fetal development comprises the gradual expansion of the embryonic body through the expansion of its compressed parts, including, most notably, the limb buds and somites.
The apparent mutual resemblance among embryos of different vertebrate groups is attributed here to the superficial similarity of the limb buds and somites: components whose shapes are established by the compression of the various forms of limbs, ribs, and vertebrae that appeared in the preceding stage.
The schematic sequences above depict the processes of morphogenesis and embryogenesis. Morphogenesis encompasses the genesis of the adult form from the regular geometrical patterns that follow from sequential subdivision of the egg (a-e; o-t). Embryogenesis constitutes the interruption of the morphogenetic sequence by the elastic recoil of the surface of the blastula that results from a split along the ventral midline (f-n). The final sequence depicts the concurrent occurrence of both processes (u-aa).
The major sequence of events occurring during gastrulation is shown in panels a-e. The upper and lower hemispheres of the ball of the blastula (a) are separated (b). The lower hemisphere is inverted (c). The upper hemisphere is inverted, turned inside out, and superimposed upon the lower hemisphere (d-e). Panels o-t show the hemispheres of the blastula undergoing gastrulation, ultimately resulting in adult morphology. Panels f-j show the dorsally directed recoil of the blastula finally reduced to the embryo form. Panels k-n show the fetal reconstruction of the pre-embryo form.
This hypothetical scheme for the development of the vertebrate body accords well with observed embryology. Moreover, it follows from the principle of condensation, where, over eons of evolutionary time, successive terminal stages of embryonic development are added while initial stages condense and eventually disappear. In the speculative scenario depicted above, the sequences referred to as embryogenesis are the observed aspects of embryology, while morphogenesis constitutes a hypothetical reconstruction of the initial stages of development lost to evolutionary condensation.
The Origin of the Notochord