In the previous chapter we saw how the simple physical assumption that the cell is a droplet of liquid comes into conflict with experimental evidence when the transport of molecules in its interior or the response of an individual cell to mechanical stress are considered. By adding more physics to the default concept of diffusion (external forces, viscosity, elasticity) we were able to approach the biological reality of cell behavior more closely. This analysis also had the premium of helping us to identify levels of organization (e.g., chemotaxis in a colony of bacteria or amoebae) at which physical laws that are too simple to explain individual cell behavior may nonetheless be relevant.
In this chapter we will describe the transition made by a developing embryo from the zygotic, or single-cell, stage to the multicellular aggregate known as the blastula. Here again the simplest physical model for both the zygote and the early multicellular embryo that arises from it is a liquid drop. As in the examples in Chapter 1 our understanding of real developing systems will be informed by an exploration of how they conform with, and how they deviate from, the basic physical picture.
The cell biology of early cleavage and blastula formation
The blastula arises by a process of sequential subdivision of the zygote, referred to as cleavage. Cleavage, in turn, is a variation on the process of cell division that gives rise to all cells.