Fig. 1, ovum; fig. 2, stage in segmentation; fig. 3, commencement of delamination after the appearance of a central cavity; fig. 4, delamination completed, mouth forming at _M._ In figs. 1, 2, and 3, _Ec._ is ectoplasm, and _En._ is endoplasm. In fig. 4, _Ec._ is epiblast, and _En._ hypoblast. _E._ and _F._ food particles.] Though embryology does not at present furnish us with a definite answer to the question how the Metazoa became developed from the Protozoa, it is nevertheless worth while reviewing some of the processes by which this can be conceived to have occurred. On purely _à priori_ grounds there is in my opinion more to be said for invagination than for any other view. On this view we may suppose that the colony of Protozoa in the course of conversion into Metazoa had the form of a blastosphere; and that at one pole of this a depression appeared. The cells lining this depression we may suppose to have been amoeboid, and to have carried on the work of digestion; while the remaining cells were probably ciliated. The digestion may be supposed to have been at first carried on in the interior of the cells, as in the Protozoa; but, as the depression became deeper (in order to increase the area of nutritive cells and to retain the food) a digestive secretion probably became poured out from the cells lining it, and the mode of digestion generally characteristic of the Metazoa was thereby inaugurated. It may be noted that an intracellular protozoon type of digestion persists in the Porifera, and appears also to occur in many Coelenterata, Turbellaria, &c., though in most of these cases both kinds of digestion probably go on simultaneously[122]. [122] J. Parker, "On the Histology of _Hydra fusca_," _Quart. Journ. Micr. Science_, vol. XX. 1880; and El. Metschnikoff, "Ueb. die intracelluläre Verdauung bei Coelenteraten," _Zoologischer Anzeiger_, No. 56, vol. III. 1880 and Lankester, "On the intracellular digestion and endoderm of Limnocodium," _Quart. Journ. Micr. Science_, vol. XXI. 1881. Another hypothetical mode of passage, which fits in with delamination, has been put forward by Lankester, and is illustrated by fig. 205. He supposes that at the blastosphere stage the fluid in the centre of the colony acquired special digestive properties; the inner ends of the cells having at this stage somewhat different properties from the outer, and the food being still incepted by the surface of the cells (fig. 205, 3). In a later stage of the process the inner portions of the cells became separated off as the hypoblast; while the food, though still ingested in the form of solid particles by the superficial cells, was carried through the protoplasm into the central digestive cavity. Later (fig. 205, 4), the point where the food entered became localised, and eventually a mouth became formed at this point.