. Text-book of embryology. Embryology. 422 INVEETEBKATA CHAP. The blastula stage may now be said to be attained. It consists of the large cell I, of six dorsal rows of three cells each, capped by three granular cells, of two ventral rows of four cells, and of an odd cell VI in the mid-ventral line. Gastrulation now begins by the retiring of the cell I from the surface. This cell may now be denominated E, as it is the mother cell of the endoderm. This cell may, however, before its inward movement begins, be divided into front and posterior cells. Whether this has occurred or not it speedily doe
. Text-book of embryology. Embryology. 422 INVEETEBKATA CHAP. The blastula stage may now be said to be attained. It consists of the large cell I, of six dorsal rows of three cells each, capped by three granular cells, of two ventral rows of four cells, and of an odd cell VI in the mid-ventral line. Gastrulation now begins by the retiring of the cell I from the surface. This cell may now be denominated E, as it is the mother cell of the endoderm. This cell may, however, before its inward movement begins, be divided into front and posterior cells. Whether this has occurred or not it speedily does divide in this way, and then by longitudinal planes into right and left halves, and then by further transverse planes, till a mass of eight cells has been produced. Then the three granular cells, if they have not divided before, now. Fig. 333.—Two views of embryos of Ocdlidina ritsseola showing tlie process of gastrulation. (After Zelinlia.) A, dorsal view of an embryo in which the endoderm cell (I = E) is just sinlting inwards. In this emhryo the granular cells ai, lt\, and IIi have each divided into right and left sister cells. Tliis is ex- ceptional. This division usually does not occur until somewhat later. B, ventral view of a somewhat later stage in the process of gastrulation. The granular cells have passed round the anterior end of the emhryo on to the ventral surface. The endoderm cell has divided into anterior (Ej) and posterior halves (E2), and is almost completely enclosed by ectoderm, gr, granular cells ; , polar body. divide each into two cells lying side by side, and the two larger rows of ventral cells are also subdivided by longitudinal planes giving rise to four rows. The six granular cells become compressed into a mass and are no longer arranged in a crescent form (Fig. 333). This mass is the rudiment of what we may term the primary stomodaeum. It becomes forced from its anterior position on to the ventral surface by the backward pressure of the cells
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