A process (gastrulation) leads to the formation of three distinct layers called germ layers: the ectoderm (outer layer), the mesoderm (middle layer), and the endoderm (inner layer).
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.
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.
We have already seen that the parenchymula stage is passed over when the gastrulation is of the invaginate type.
Gastrulation takes place by epiboly, and the stomodaeum (oral invagination - mastax pharynx) takes place in two stages of the region of the closed blastopore.
Then follows the phenomenon of gastrulation, by which onehalf of the blastula is invaginated into the other, so as to obliterate the segmentation cavity.
P. Hazen, "The Gastrulation of Amphioxus," J.
(1900), p. 569; P. Sammassa, "Studien Aber den Einfluss des Dotters auf die Gastrulation and die Bildung der primaren Keimblatter der Wirbelthiere: iv.