The differentiation of the epiblast and hypoblast may commence during the later phases of the segmentation, but is generally not completed till after its termination. Not only do the cells of the blastoderm become differentiated into two layers, but these two layers, in the case of a very large number of ova with but little food-yolk, constitute a double-walled sack--the gastrula (fig. 198)--the characters of which are too well known to require further description. Following the lines of phylogenetic speculation above indicated, it may be concluded that the two-layered condition of the organism represents in a general way the passage from the protozoon to the metazoon condition. It is probable that we may safely go further, and assert that the gastrula reproduces, with more or less fidelity, a stage in the evolution of the Metazoa, permanent in the simpler Hydrozoa, during which the organism was provided with (1) a fully developed digestive cavity (fig. 198 _b_) lined by the hypoblast with digestive and assimilative functions, (2) an oral opening (_a_), and (3) a superficial epiblast (_d_). These generalisations, which are now widely accepted, are no doubt very valuable, but they leave unanswered the following important questions: (1) By what steps did the compound Protozoon become differentiated into a Metazoon? (2) Are there any grounds for thinking that there is more than one line along which the Metazoa have become independently evolved from the Protozoa? (3) To what extent is there a complete homology between the two primary germinal layers throughout the Metazoa? [FIG. 198. DIAGRAM OF A GASTRULA. (From Gegenbaur.) _a._ mouth; _b._ archenteron; _c._ hypoblast; _d._ epiblast.] Ontogenetically there is a great variety of processes by which the passage from the segmented ovum to the two-layered or diploblastic condition is arrived at. These processes may be grouped under the following heads: 1. Invagination. Under this term a considerable number of closely connected processes are included. When the segmentation results in the formation of a blastosphere, one half of the blastosphere may be pushed in towards the opposite half, and a gastrula be thus produced (fig. 199, A and B). This process is known as embolic invagination. Another process, known as epibolic invagination, consists in epiblast cells growing round and enclosing the hypoblast (fig. 200). This process replaces the former process when the hypoblast cells are so bulky from being distended by food-yolk that their invagination is mechanically impossible. [FIG. 199. TWO STAGES IN THE DEVELOPMENT OF HOLOTHURIA TUBULOSA, VIEWED IN OPTICAL SECTION. (After Selenka.) A. Stage at the close of segmentation. B. Gastrula stage. _mr._ micropyle; _fl._ chorion; _s.c._ segmentation cavity; _bl._ blastoderm; _ep._ epiblast; _hy._ hypoblast; _ms._ amoeboid cells derived from hypoblast; _a.e._ archenteron.]