At the time the mesoblast is first formed the hypoblast cells, which roof the mesenteron, are often imperfectly two layers thick (fig. 38). They soon however become constituted of a single layer only. When the mesoblast is fairly established, the lateral parts of the hypoblast grow inwards underneath the axial part, so that the latter (fig. 39, _ch_) first becomes isolated as an axial cord, and is next inclosed between the medullary cord (_nc_) (which has by this time been formed) and a continuous sheet of hypoblast below (fig. 40). Here its cells divide and it becomes the notochord. The notochord is thus bodily formed out of the axial portion of the primitive hypoblast. Its mode of origin may be compared with that in Amphioxus, in which an axial fold of the archenteric wall is constricted off as the notochord. The above features in the development of the notochord were first established by Calberla[27] (No. 78). [27] In Calberla's figure, shewing the development of the notochord, the limits of mesoblast and hypoblast are wrongly indicated. [FIG. 40. TRANSVERSE SECTION THROUGH PART OF AN EMBRYO OF PETROMYZON PLANERI OF 256 HOURS. _m.c_. medullary cord; _ch_. notochord; _al_. alimentary canal; _ms_. mesoblastic plate.] _General history of the development._ Up to about the time when the enclosure of the hypoblast by the epiblast is completed, no external traces are visible of any of the organs of the embryo; but about this time, _i.e._ about 180 hours after impregnation, the rudiment of the medullary plate becomes established, as a linear streak extending forwards from the blastopore over fully one half the circumference of the embryo. The medullary plate first contains a shallow median groove, but it is converted into the medullary cord, not in the usual vertebrate fashion, but, as first shewn by Calberla, in a manner much more closely resembling the formation of the medullary cord in Teleostei. Along the line of the median groove the epiblast becomes thickened and forms a kind of keel projecting inwards towards the hypoblast (fig. 39, _nc_). This keel is the rudiment of the medullary cord. It soon becomes more prominent, the median groove in it disappears, and it becomes separated from the epiblast as a solid cord (fig. 40, _mc_). By this time the whole embryo has become more elongated, and on the dorsal surface is placed a ridge formed by the projection of the medullary cord. At the lip of the blastopore the medullary cord is continuous with the hypoblast, thus forming the rudiment of a neurenteric canal. Calberla gives a similar account of the formation of the neural canal to that which he gives for the Teleostei (_vide_ p. 72).