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.
Fig. 57. Transverse section through the ovum of Euaxes during an early stage of development. (After Kowalevsky.)
ep. epiblast; ms. mesoblastic band; hy. hypoblast.
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. 58. Two stages in the development of Stephanomia pictum.
(After Metschnikoff.)