Noun:

கருவளர்ச்சியின் ஒருநிலை,

gastrula's Usage Examples:

This is a process of gastrulation by invagination which is found in all animals above the Coelenterata, but which is very rare in the Cnidaria, and is known only in the Scyphomedusae amongst the Hydrozoa.

The term " diblastula " was subsequently adopted in England for the gastrula of Haeckel.

It should be added that the process of development, the gastrulation as it is termed, may be shortened by the immigration of cells taking place C From Balfour, after Kowalewsky.

After the gastrula stage, which is found as a developmental stage in all Enterozoa, the embryo of the Hydrozoa proceeds to develop characters which are peculiar to the Coelen- a terata only.

After the formation of an invaginate gastrula the larval form is rapidly acquired.

While still within the egg-membrane the epiblastic cells become flagellated, and the gastrula rotates within the membrane.

If the embryo is set free as a free-swimming, so-called planula-larva, in the blastula, parenchymula, or gastrula stage, then a free actinula stage is not found; if, on the other hand, a free actinula occurs, then there is no free planula stage.

c, Nearly ripe cercariae; cc, cystogenous cells; dr, daughter-redia; dt, limbs of the digestive tract; f, head-papilla; h, eye-spots; h', same degenerating; k', germinal cell; 1, cells of the anterior row; m, embryo in optical section, gastrula stage; n, pharynx of redia; o, digestive sac; oe, oesophagus.

To the primitive two-cell-layered form, the hypothetical ancestor of all Metazoa or Enterozoa, Haeckel gave the name Gastraea; the em- bryonic form which represents in the individual growth from the egg this ancestral condition he called a " gastrula.

The gastrula is formed in this case by invagination.

It has usually been regarded as representing both endoderm and mesoderm, and the groove which usually leads to its formation has been compared to the abnormally elongated blastopore of a typical gastrula.

If the three principal organ-systems of the medusa, namely mouth, tentacles and umbrella, be considered in the light of phylogeny, it is evident that the manubrium bearing the mouth must be the oldest, as representing a common property of all the Coelentera, even of the gastrula embryo of all Enterozoa.

"It has usually been regarded as representing both endoderm and mesoderm, and the groove which usually leads to its formation has been compared to the abnormally elongated blastopore of a typical gastrula.