The Questions of Developmental Biology

15. Lateral plate mesoderm and endoderm
In the chaos of the English civil wars, William Harvey, physician to the King and discoverer of the
blastoderm, was comforted by viewing the heart as the undisputed ruler of the body, through whose
divinely ordained powers the lawful growth of the organism was assured. Later embryologists looked at the
heart as more of a servant than a ruler, the chamberlain of the household who assured that the nutrients
reached the centrally located brain and peripherally located muscles. In either metaphor, the heart, its
circulation, and the digestive system were seen as being absolutely critical during development. As Harvey
(1651) persuasively argued, the chick embryo must form its own blood without any help from the hen, and
this blood is crucial in embryonic growth. How this happened was a mystery. "What artificer," he wrote,
could create blood "when there is yet no liver in being?" The nutrition provided by the egg was also
paramount to Harvey. His conclusion about the nutritive value of the yolk and albumin was that "The egge
is, as it were, an exposed womb; wherein there is a substance concluded as the Representative and
Substitute, or Vicar of the breasts."
This chapter will outline the mechanisms by which the circulatory system, the respiratory system,
and the digestive system emerge in the amniote embryo.
Lateral Plate Mesoderm
On either side of the intermediate mesoderm resides the lateral plate mesoderm. Each
plate splits horizontally into the dorsal somatic (parietal) mesoderm, which underlies the
ectoderm, and the ventral splanchnic (visceral) mesoderm, which overlies the endoderm.
The space between these layers
becomes the body cavity the
coelom which stretches from the
future neck region to the posterior of
the body. During later development, the
right- and left-side coeloms fuse, and
folds of tissue extend from the somatic
mesoderm, dividing the coelom into
separate cavities. In mammals, the
coelom is subdivided into the pleural,
pericardial, and peritoneal cavities,
enveloping the thorax, heart, and
abdomen, respectively. The mechanism
for creating the linings of these body
cavities from the lateral plate
mesoderm has changed little throughout
vertebrate evolution, and the development of the chick mesoderm can be compared with similar
stages of frog embryos (Figure 15.1).