Fig. 201. Two stages in the development of Stephanomia pictum, to illustrate the formation of the layers by delamination. (After Metschnikoff.)
A. Stage after the delamination; ep. epiblastic invagination to form pneumatocyst.
B. Later stage after the formation of the gastric cavity in the solid hypoblast. po. polypite; t. tentacle; pp. pneumatocyst; ep. epiblast of pneumatocyst; hy. hypoblast surrounding pneumatocyst.
b. Delamination where the segmented ovum has the form of a blastosphere, the cells of which give rise by budding to scattered cells in the interior of the vesicle, which, though they may at first form a solid mass, finally arrange themselves in the form of a definite layer around a central digestive cavity ([fig. 202]).
c. Delamination where the segmented ovum has the form of a blastosphere in the cells of which the protoplasm is differentiated into an inner and an outer part. By a subsequent process the inner parts of the cells become separated from the outer, and the walls of the blastosphere are so divided into two distinct layers ([fig. 205]).
Although the third of these processes is usually regarded as the type of delamination, it does not, so far as I know, occur in nature, but is most nearly approached in Geryonia ([fig. 203]).
The first type of delamination is found in the Ceratospongiæ, some Silicispongiæ (?), and in many Hydrozoa and Actinozoa, and in Nemertea and Nematelminthes (Gordioidea?). The second type occurs in many Porifera [Calcispongiæ (Ascetta), Myxospongiæ], and in some Cœlenterata, and Brachiopoda (Thecidium).
Fig. 202. Three larval stages of Eucope polystyla. (After Kowalevsky.)
A. Blastosphere stage with hypoblast spheres becoming budded off into central cavity. B. Planula stage with solid hypoblast. C. Planula stage with a gastric cavity. ep. epiblast; hy. hypoblast; al. gastric cavity.
Delamination and invagination are undoubtedly the two most frequent modes in which the layers are differentiated, but there are in addition several others. In the first place the whole of the Tracheata (with the apparent exception of the Scorpion) develop, so far as is known, on a plan peculiar to them, which approaches delamination. This consists in the appearance of a superficial layer of cells enclosing a central yolk mass, which corresponds to the hypoblast ([figs. 204] and [214]). This mode of development might be classed under delamination, were it not for the fact that the early development of many Crustacea is almost the same, but is subsequently followed by an invagination ([fig. 208]), which apparently corresponds to the normal invagination of other types. There are strong grounds for thinking that the tracheate type of formation of the epiblast and hypoblast is a secondary modification of an invaginate type (vide Vol. II. p. 457).