Embryonic DevelopmentVARIATIONS IN EMBRYONIC GERM LAYERS AND BODY CAVITY
Formation of the Digestive Cavity
At the Blastula stage, the embryo forms a fluid filled ball of cells
During Gastrulation, the blastula caves in on one end to form a Gastrula. The cavity formed by gastrulation will form a digestive cavity or digestive tract.
If gastrulation is incomplete, the gastric cavity will have only one opening. This arrangement is characteristic of Cnideria and Platyhelminthes
If gastrulation is complete, the digestive tract will have two openings. This arrangement has 2 variations, protostomes and deuterostomes
Protostomes vs. Deuterostomes
In Protostomes, the blastopore forms the mouth. In Deuterostomes, the blastopore forms the anus and the mouth is the second opening
Embryonic Germ Layers
Organisms that undergo gastrulation fit into 2 categories: diploblasts and triploblasts
Diploblasts only have 2 embryonic germ layers
Gastrulation forms an inner layer of cells (endoderm) and an outer layer (ectoderm)
This diagram shows the endoderm in blue and the ectoderm in black
Embryonic Germ Layers
Triploblasts develop a third germ layer in between the endoderm and the ectoderm
The middle layer is called mesoderm In the diagram, mesoderm is
represented in yellow Variations in the presence, absence
and arrangement of mesodermal tissue is one of the most important distinguishing features of animal phyla
Phylogeny based on Mesodermal Arrangement
Phylum porifera lack a true digestive cavity
Cnideria are diploblastic. They have a digestive cavity but lack mesodermal tissue
The triploblastic phyla vary in regard to the arrangement of mesoderm and body cavity
Development of Mesodermal Tissue
Mesoderm is formed by a migration of endodermal cells into the blastocoel.
Mesoderm forms by one of 2 possible mechanism
The diagram at the right shows mesoderm development in acoelomates and protostomes
The formation of mesoderm initiates near the blastopore. Dividing mesodermal cells migrate into the blastocoel
Acoelomate phyla
Acoelomates do not form a body cavity
Flatworms (Platyhelminthes) are acoelomate
Mesodermal cells fill the entire blastocoel
Internal organs form, but are not separate and distinct
Pseudocoelomate Phyla
Pseudocoelomates have a body cavity, but mesoderm is associated with the outer body wall (ectoderm).
Nematodes and rotifers are pseudocoelomate
All pseudocoelomates have a true digestive tract (2 openings) and are protostomes (blastopore = mouth)
The digestive tract lacks mesodermal tissue, thus is not muscular
Cross Section of a Pseudocoelomate
The diagram shows a cross section through a nematode
Notice the space between organs (body cavity), but note that there is only mesoderm on the outer boundary, associated with the body wall. The Intestine is a simple layer of epithelium (no muscle)
The organs are separate and distinct, but the body cavity is “false” – not fully lined with mesoderm
Coelomate Protostomes
A true body cavity is fully lined with mesodermal tissue.
In coelomate protostomes, the body cavity is “schizocoelous” (note the descriptive root word “schizo”)
The solid mass of mesoderm filling the blastocoel splits due to the programmed cell death (“apoptosis”) of some centrally located cells
The diagram shows the coelom in pink
Coelomate Deuterostomes
All deuterostomes have a true body cavity
In deuterostomes, the blastopore forms the anus (so the “second opening” forms the mouth”)
In deuterostomes, the mesoderm forms from outpouching of the gut
Since it is “caving in” it is “enterocoelous”
Because the coelom is formed from pouches, it is completely surrounded by mesodermal tissue
Schizocoelous vs Enterocoelous
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