### physical process | biological process:

Gastrulation
* adds more cells to the embryo and sorts the cells into distinct cell layers.
* are part of growth.
  * varies in different phyla. It is followed by 'organogenesis', when individual organs develop within the newly formed 'germ layers'.
* begins after egg laying
  - three hours after fertilization
  - when fluid starts to accumulate between the blastomeres of the morula
* complicated process in all animals and is difficult to define and study.
* involves an epithelial to mesenchymal transformations
  - cell movements that help to attain new shape and morphology of the embryo
  - changes in cell motility, cell shape, and cell adhesion
  - the rearrangement and migration of cells from the epiblast
* is apparently by epiboly.
* is by a combination of invagination and epiboly
  - epiboly and invagination
  - epiboly, followed by invagination
  - invagination, occasionally accompanied by an in-wandering of cells
* is initiated at the future dorsal side of the embryo, underneath the organiser region
  - by invagination of cells to form the dorsal lip of the blastopore
  - probably by epiboly
* is the defining event of gut morphogenesis
  - process by which presumptive endoderm and mesoderm are internalized
* leads to the development of different germ layers in the embryo.
* marks the onset of changes in cell behavior that begin to shape the organism.
* occurs before organogenesis
  - between the sixth and seventh cleavages
* occurs by ingression and is followed by the deposition of the thick cuticle
  - unipolar proliferation
  - differently in different animals
  - in roughly two forms, depending on the amount of yolk present
  - solely on the blastodisc
* produces a gastrula that develops three embryonic germ layers.
* reorganizes the blastula into a three-layered embryo with a recognizable body plan.
* represents a critical time in embryogenesis.
* starts in the so called Spemann organizer.

Hemopoiesis
* All hemopoiesis begins with hemocytoblasts.
* biological process

Internal respiration
* includes exchanges.
* is exchange of gases between blood in systemic capillaries and tissue fluid.
* is the exchanging of gases at the cellular level
  - gas exchange between the blood and tissue cells
* occurs at the systemic capillaries within the tissues of the body.

### physical process | biological process:

Involution
* are construction
  - useful when dealing with images of symmetrical objects
* is the curling inward and ingrowth of a group of cells
  - migration of single cells from an epithelial layer into an embryonic cavity
* is the process by which actuality is infolded into latency
  - where a hemangioma begins to lighten and become less prominent
* normal process.
* produces the germ ring by folding the blastoderm back upon itself.
* results in a decrease in the number of secretory cells in the mammary gland tissue.

Keratinization
* contribute to the pale pink color with a bluish hue, indicating subtle vascularity.
* feature of squamous cell carcinomas.
* is one of the diagnostic features of squamous cell carcinoma
  - the process by which keratin is produced and the stratum corneum is formed
  - typical of epidermis