Building Vertebrate Embryos: Heads and Tails

Abstract

Differentiation and morphogenesis of the soma of vertebrate embryos is epigenetic and hierarchical. Early events, as determined by experimental investigation of amphibian embryos, introduced in section 8.3 and discussed in this chapter, are:

• 1. establishment of the primary embryonic axis at the time of sperm entry into the egg;

• 2. dorsalization of the zygote and establishment of the D-V axis (ss. 9.4, 9.5);

• 3. induction and specification of the mesoderm (ss. 10.1-10.3);

• 4. differentiation of the chordamesoderm (the future notochord);

• 5. induction of neural ectoderm from presumptive ectoderm by the chordamesoderm;

• 6. establishment of the neural axis and morphogenesis of the neural tube (s. 10.2);

• 7. origin of the neural crest at the boundary between neural and epidermal ectoderm (s. 10.4); and

• 8. a process of secondary neurulation and tailbud formation that is fundamentally different from how cranial and trunk regions form.¹

‘Development is therefore the immediate cause of introduction of variation at the level of the individual organism … Central to understanding the role of development in evolutionary mechanisms must be the study of the emergent and epigenetic properties of developing systems and their unique role in the processes by which variation is introduced among individual phenotypes.’ Thomson, 1988, pp. 16-17