GASTRULATION ItEcossmERED. Three principal types of gastrulation are recognized among ani mals: (a.) Invagination, which, as the typical process, has been already referred to. In partial cleavage the yolk-laden cells cannot be pushed into the small cleavage cavity, and under these circumstances the ectoderm grows over the entoderm and gradually surrounds it completely; this process is known as `epiboly,' in contrast to the typical embolic invagination that occurs in alecithal eggs. (b.) Delamination. The cells of the one-layered germ divide into a superficial and a deep-lying layer of cells. The latter be come the entoderm. This type of gastrulation occurs in sponges, various coelenterates. and some of the lower worms. (c.) Ingression. At one pole of the one-layered germ, cells begin to migrate into the blastula cavity to form the inner layer.
This process is found especially in hydroids. The aim of all sorts of gastrulation is the establish ment of a two-layered germ containing a diges tive cavity. (d.) Formation of mesoderm. The mesoderm arises in a great number of different as a pair of pouches or a series of paired pouches from the entoderm, sometimes from primitive `mesoblast' cells which arise very early; sometimes split off from the other germ-layers. Indeed, the varied origin of the mesoderm, despite the similarity hf the or gans to which it gives rise, is one of the strong est hits of evidence of the inadequacy of the eell layer doctrine.
It will not be practicable to go into the de velopmental history of all the groups. We will. however, consider in some detail the development of the chick. For the development of man. see