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. pneumatophore; ep. epiblastic invagination to form pneumatocyst; hy. hypoblast surrounding pneumatocyst.
In delamination, when the segmentation is not uniform, or when a solid morula is formed, the differentiation of the epiblast and hypoblast is effected by the separation of the central solid mass of cells from the peripheral cells ([fig. 58] A).
In the case of epibolic invagination as well as in that of the type of delamination just spoken of, the archenteric cavity is in most cases secondarily formed in the solid mass of hypoblast ([fig. 58] B).
In ova with a partial segmentation there is usually some modification of the epibolic gastrula.
Many varieties are found in the animal kingdom of the types of invagination and delamination just characterized, and in not a few forms the layers originate in a manner which cannot be brought into connection with either of these processes.
Fig. 59. Epibolic Gastrula of Bonellia. (After Spengel.)
A. Stage when the four hypoblast cells are nearly enclosed.
B. Stage after the formation of the mesoblast has commenced by an infolding of the lips of the blastopore.
ep. epiblast; me. mesoblast; bl. blastopore.