Triploblasty

{{short description|State of having three germ layers in embryonic development}}

File:Figure 27 02 05.jpg

Triploblasty is a condition of the gastrula in which there are three primary germ layers: the ectoderm, mesoderm, and endoderm. Germ cells are set aside in the embryo at the blastula stage, and are incorporated into the gonads during organogenesis. The germ layers form during the gastrulation of the blastula. The term triploblast may refer to any egg cell in which the blastoderm splits into three layers.{{cite book|last=Ravichandra|title=Plant Nematology|date=Jan 1, 2008|publisher=I. K. International Pvt Ltd|pages=29|url=https://books.google.com/books?id=JPMPM3PRbQwC&q=the+term+may+refer+to+an+ovum+in+which+the+blastoderm+splits+into+three+layers&pg=PA29|isbn=9788189866617}}

All bilaterians, which are the animals with bilaterally symmetrical embryos, are triploblastic. Other animal taxa, namely the ctenophores, placozoans, and cnidarians, are diploblastic, which means that their embryos contain only two germ layers. Sponges are even less developmentally specialized, because they lack both true tissues{{cite web

|url=https://www.qm.qld.gov.au/microsites/biodiscovery/03sponges-and-corals/structure-of-sponges.html

|title=Structure of Sponges

|last=Hooper

|first=John

|year=2018

|website=Queensland Museum

|access-date=27 September 2019

|archive-url=https://web.archive.org/web/20190926205031/https://www.qm.qld.gov.au/microsites/biodiscovery/03sponges-and-corals/structure-of-sponges.html

|archive-date=26 September 2019

|url-status=dead

}} and organs.{{cite journal

| last1=Thacker

| first1=Robert W

| last2=Diaz

| first2=Maria Christina

| date=8 September 2014

| title= The Porifera Ontology (PORO): enhancing sponge systematics with an anatomy ontology

| journal=J Biomed Semantics

| volume=5

| issue=39

| pages=39

| doi=10.1186/2041-1480-5-39

| pmc=4177528

| pmid=25276334

| doi-access=free

}}

The earliest triploblasts are thought to have evolved from the diploblasts at some time during the Proterozoic era, establishing themselves as a group prior to their diversification during the Cambrian explosion.{{cite journal |last1=Gu |first1=Xun |title=Early metazoan divergence was about 830 million years ago |journal=Journal of Molecular Evolution |date=1998 |volume=47 |issue=3 |pages=369–371 |doi=10.1007/PL00013150 |pmid=9732464 |bibcode=1998JMolE..47..369G |s2cid=30228208 }}{{cite journal |last1=Lynch |first1=Michael |title=The age and relationships of the major animal phyla |journal=Evolution |date=1999 |volume=53 |issue=2 |pages=319–325 |doi=10.1111/j.1558-5646.1999.tb03768.x |pmid=28565434 |s2cid=6120649 |doi-access=free }}{{cite journal |last1=Seilacher |first1=Adolf |last2=Pradip K. |first2=Bose |last3=Pflüger |first3=Friedrich |title=Triploblastic animals more than 1 billion years ago: trace fossil evidence from India. |journal=Science |date=1998 |volume=282 |issue=5386 |pages=80–83 |doi=10.1126/science.282.5386.80 |pmid=9756480 |bibcode=1998Sci...282...80S }}