Vertebrate Development

Today we know that ontogeny does not exactly recapitulate phylogeny, nevertheless, an individual's ontogeny or development constrains, or sometimes opens evolutionary pathways. For this reason, we must establish some basic knowledge about chordate development. We start with the zygote or fertilized ovum, the individual's first manifestation.

Cleavage from A. S. Romer. 1977. The Vertebrate Body.
Consider the basic steps by which we get a proper embryo with a top, bottom, front, and back from a zygote:

Polarity: In all ova yolky matrial tends to concentrate at one end, yielding:

The amount of yolk greatly influences developmental dynamics. For now, we consider an ovum with relatively little yolk.

Cleavage Phase of rapid cell division with little overall growth. The zygote transforms into a hollow sphere of cells, the blastula. The space in the middle is the blastocoel

The blastula has two cell types:

Gastrulation from A. S. Romer. 1977. The Vertebrate Body.

The embryo is now a gastrula. It possesses these basic germ layers: The blastopore becomes an opening from the gut to the outside. In cnidarians and ctenophores, this is simple, as there is only one "mouth." In bilaterians, the blastopore will become either the mouth or the anus, depending on the taxon.

In cnidarians, the blastula assumes the form of the planktonic planula larva which then directly develops features of an adult with no front, back, left, or right. For bilaterians, however, it is more complicated.

Bilaterian Development:

Mesoderm: As the illustration indicates, a third basic cell type, mesoderm, is present in the blastula, forming a collar around the large cells of the vegetal pole. They are fated to give rise to a third germ layer. Like the endoderm, they invaginate into the interior, but do so asymmetrically, extending along one side of the archenteron. This extension marks the animal's plane of bilateral symmetry. From there, mesoderm cells proliferate into the space between the endoderm and ectoderm, giving rise to a great range of three-dimensional structures and organs.

Eucoelomate coelom schematic from A Review of the Universe
The Coelom: A characteristic feature of bilaterians is the presence of a coelom or body cavity. This feature allows for:

Eucoelomate coelom schematic from Fueleducation
The evolution of the coelom opened many vistas for animal evolution, including significantly expanded locomotor strategies. Cnidarians show the limits of what a hydrostatic skeleton can do for an animal with a single module. Bilaterians, however, display body segmentation in which separate modules of the hydrostatic skeleton can lengthen and shorten, facilitating much more complex movement. This makes possible activities like: Indeed, non-bilaterians are deemed incapable of burrowing.

Specialized organs: These capabilities came at a price. Animals with only endoderm and ectoderm don't need to worry about gas exchange and elimination of nitrogenous waste, because no living cell is so far from the body surface that simple diffusion can't do the trick. Bilaterians, in contrast, usually require specialized organs for functions like:

Fortunately, the presence of mesoderm and a coelom seems to bestow the developmental plasticity needed to allow these to evolve. Indeed, the gut tube, kidneys, and gonads are ancestrally suspended inside the coelom.

Protostomous and deuterostomous development from Nature Volume 409 Number 6816

Deuterostome Development:

Bilateria breaks down into two major groups that were first distinguished by developmental characters (although molecular phylogenies have supported them):

Protostomia: Includes arthropods, mollusks, annelid worms, brachiopods, and bryozoans plus some minor groups. Synapomorphies include:

Deuterostomia: Includes chordates and echinoderms plus some minor groups. Synapomorphies include:

Neurulation in Branchiostoma from A. S. Romer. 1977. The Vertebrate Body.

Chordate Development:

Neurulation: The indeterminate nature of deuterostome development, in which the fates of specific cells are influenced by inductive relationships with other cells, is illustrated by the next big step chordate development - the formation of the neural tube that gives rise to the central nervous system. In this process, the activity of mesoderm cells triggers a cascade of events. At this stage we have three distinct flavors of ectoderm: There is an extra twist: Distinct regions of the external ectoderm of the head thicken to form placodes. These interact with outpouchings of the neural tube ectoderm to form the special sense organs.

Mesoderm schematic from Studyblue
Stupid mesoderm tricks: While neurulation is happening, mesoderm is also busy. Aside from the gut tube (endoderm), skin, and nervous system (ectoderm), most of the body derives from mesoderm. Here are a few major features: