When segmentation does not take place on the regular type the processes above described are as a rule somewhat modified. The yolk is usually concentrated in the cells which would, in the case of a simple gastrula, be invaginated. As a consequence of this, these cells become (1) distinctly marked off from the epiblast cells during the segmentation; and (2) very much more bulky than the epiblast cells. The bulkiness of the hypoblast cells necessitates a modification of the normal process of embolic invagination, and causes another process to be substituted for it, viz. the growth of the epiblast cells as a thin layer over the hypoblast. This process (fig. 57) is known as epibolic invagination. The point where the complete enclosure of the hypoblast cells is effected is known as the blastopore. All intermediate conditions between epibolic and embolic invagination have been found. [FIG. 57. TRANSVERSE SECTION THROUGH THE OVUM OF EUAXES DURING AN EARLY STAGE OF DEVELOPMENT. (After Kowalevsky.) _ep._ epiblast; _ms._ mesoblastic band; _hy._ hypoblast.] [FIG. 58. TWO STAGES IN THE DEVELOPMENT OF STEPHANOMIA PICTUM. (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._ 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.]