The processes by which the germinal layers take their origin are largely influenced by the character of the segmentation, which, as was shewn in the last chapter, is mainly dependent on the distribution of the food-yolk. When the segmentation is regular, and results in the formation of a blastosphere, the epiblast and hypoblast are usually differentiated from the uniform cells forming the wall of the blastosphere in one of the two following ways. (1) One-half of the blastosphere may be pushed in towards the other half. A two-layered hemisphere is thus established which soon elongates, while its opening narrows to a small pore (fig. 55). The embryonic form produced by this process is known as a gastrula. The process by which it originates is known as embolic invagination, or shortly invagination. Of the two layers of which it is formed the inner one (_c_) is known as the hypoblast and the outer (_d_) as the epiblast, while the pore leading into its cavity lined by the hypoblast is the blastopore (_a_). The cavity itself is the archenteron (_b_). (2) The cells of the blastosphere may divide themselves by a process of concentric splitting into two layers (fig. 56, 3). The two layers are as before the epiblast and hypoblast, and the process by which they originate is known as delamination. The central cavity or archenteron (_F_) is in the case of delamination the original segmentation cavity; and not an entirely new cavity as in the case of invagination. By the perforation of the closed two-walled vesicle resulting from delamination an embryonic form is produced which cannot be distinguished in structure from the gastrula produced by invagination (fig. 56, 4). The opening (_M_) in this case is not however known as the blastopore but as the mouth. [FIG. 56. DIAGRAM SHEWING THE FORMATION OF A GASTRULA BY DELAMINATION. (From Lankester.) 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 fig. 1, 2 and 3 _Ec._ is ectoplasm, and _En._ is entoplasm. In fig. 4 _Ec._ is epiblast and _En._ hypoblast.]