If the preparation is preserved for several hours longer, we may see the cells formed by segmentation unite around the circumference so as to form a hollow bag corresponding to a gastrula, as shown in Fig.
"The Mechanism of Life"
Stéphane Leduc
This will represent the two-layered "gastrula"-the simplest ancestral form of the Metazoa: a form which is permanently represented in some of the lowest types; for it needs but tentacles round the mouth of the sac, to produce a common hydra.
"Essays: Scientific, Political, & Speculative, Vol. I"
Herbert Spencer
The gastrula stage in the development of embryos, where by invagination such an arrangement of the multiplying cells is secured as to offer the greatest possible surface consistent with a first division of labour; the provision of cilia for drawing upon the energy supplies of the medium; and more generally the specialisation of organs in the higher developments of life, may alike be regarded as efforts of the organism directed to the absorption of energy.
"The Birth-Time of the World and Other Scientific Essays"
J. (John) Joly