Final answer:
The major form of programmed cell death during development is apoptosis, a necessary process for the proper formation of body structures and for maintaining the health of the organism by eliminating damaged or unnecessary cells.
Step-by-step explanation:
The major form of programmed cell death during development is apoptosis. Apoptosis is the process by which cells that are no longer necessary, damaged, or potentially dangerous efficiently and neatly self-destruct. This is a critical mechanism during various stages of development, such as the formation of the body's structures. For instance, in vertebrate embryonic development, apoptosis helps in the removal of web-like tissue between fingers and toes, allowing for properly separated digits. Additionally, apoptosis plays a vital role in the immune system, where it aids in the elimination of T-cells that mistakenly recognize self proteins, preventing autoimmune diseases.
Conversely, necrosis is a form of cell death that results from external injury or trauma rather than a programmed sequence. Autophagy is a process where a cell degrades its own components as a survival mechanism, typically under conditions of nutrient deprivation. Cytolysis is the bursting of a cell due to an influx of water, and complete transcriptional shutdown is not a form of cell death but rather a halt in the cell's genetic expression processes.