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.
Then follows the phenomenon of gastrulation, by which onehalf of the blastula is invaginated into the other, so as to obliterate the segmentation cavity.
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.