Embryo Development

Intro

A developing organism follows these steps in order like dominos falling, each proceeding event is the trigger for the next event: Cell proliferation, specialization/differentiation, movement, and then tissue shaping/morphogenesis.

#1

Egg is fertilized

#2

Cells then begin to divide, called cleavage. This changes the number of cells, not the size of the cells (blastula and fertilized egg are the same size).

#3

Symmetrical blastula forms

#4

Blastula turns inside out, called gastrulation. This is the initial differentiation of cells plus movement to different areas. The cell increases in size with more types of cells forming and matriculating in specific environments. 

• Cell types mentioned include ectoderm, endoderm, and mesoderm.

#5

Formation of the gastrula. The ectoderm cells form the outside of the gastrula, while the endoderm creates the inner section of the embryo (primordial gut). Mesoderm differentiated cells are scattered between the ectoderm and endoderm cell layers so that they are able to form a variety of tissues and organs. 

#6

The gastrula is not symmetric; this gives rise to axis specification. This is clear directionality of the developing embryo. The dorsal/ventral region separates the top and bottom of the embryo, the anterior/posterior region differentiates the front and the back of the embryo, and then there is a left and right axis separating the embryo.

Conclusion

Overall, the basic body plan is established very early in development and it persists for the lifespan of the organism.