While the above changes are taking place, the small cells, or as they may now be called the epiblast cells, gradually spread over the large yolk-cells, as in normal types of epibolic invagination. The growth over the yolk-cells is not symmetrical, but is most rapid in the meridian opposite the opening of the alimentary cavity, so that the latter is left in a bay (cf. Elasmobranchii, p. 63). The epibolic invagination takes place as in Molluscs and many other forms, not simply by the division of pre-existing epiblast cells, but by the formation of fresh epiblast cells from the yolk-cells (fig. 37); and till after the complete enclosure of the yolk-cells there is never present a sharp line of demarcation between the two groups of cells. By the time that the segmentation cavity is obliterated the whole yolk is enclosed by the epiblast. The yolk-cells adjoining the opening of the mesenteron are the latest to be covered in, and on their enclosure this opening constitutes the whole of the blastopore. The epiblast is composed of a single row of columnar cells. Mesoblast and notochord. During the above changes the mesoblast becomes established. It arises, as in Elasmobranchs, in the form of two plates derived from the primitive hypoblast. During the invagination to form the mesenteron some of the hypoblast cells on each side of the invaginated layer become smaller, and marked off as two imperfect plates (fig. 38, _ms_). It is difficult to say whether these plates are entirely derived from invaginated cells, or are _in part_ directly formed from the pre-existing yolk-cells, but I am inclined to adopt the latter view; the ventral extension of the mesoblast plates undoubtedly takes place at the expense of the yolk-cells. The mesoblast plates soon become more definite, and form (fig. 39, _ms_) well-defined structures, triangular in section, on the two sides of the middle line. [FIG. 39. TRANSVERSE SECTION THROUGH AN EMBRYO OF PETROMYZON PLANERI OF 208 HOURS. The figure illustrates the formation of the neural cord and of the notochord. _ms._ mesoblast; _nc._ neural cord; _ch._ notochord; _yk._ yolk-cells; _al._ alimentary canal.]