_as´´´._ medulla oblongata; _is^x._ splanchnopleure; _as^x._ somatopleure in the vertebral part of the mesoblastic plate; _s._ lateral plate of mesoblast; _f._ throat; _e._ passage of epithelial cells into yolk-cells; _d._ yolk-cells; _r._ dorsal groove along the line of junction of the medullary folds.] After the formation of the notochord (fig. 72), the mesoblast may be regarded as consisting of two lateral plates, continuous ventrally, but separated in the median dorsal line. By the division of the dorsal parts of these plates into segments, which commences in the region of the neck and thence extends backwards, the mesoblast of the trunk becomes divided into a vertebral portion, cleft into separate somites, and a lateral unsegmented portion (fig. 74). The history of these two parts and of the mesoblast is generally the same as in Elasmobranchs. The mesoblast in the head becomes, according to Götte, divided into four segments, equivalent to the trunk somites. Owing to a confusion into which Götte has fallen from not recognizing the epiblastic origin of the cranial nerves, his statements on this head must, I think, be accepted with considerable reserve; but some part of his segments appears to correspond with the head-cavities of Elasmobranchii. Urodela. Scott and Osborn (No. 114) have shewn that in the Newt the mesoblast (fig. 75) is formed of two lateral plates, split off from the hypoblast, and that the ventral growth of these plates is largely effected by the conversion of yolk-cells into mesoblast-cells. They have further shewn that the notochord is formed of an _axial portion of the hypoblast_, as in the types already considered (fig. 75). The body cavity is continued into the region of the head; and the mesoblast lining the cephalic section of the body cavity is divided into the same number of head cavities as in Elasmobranchii, viz. one in front of the mouth, and one in the mandibular and one in each of the following arches. The hypoblast. There are no important points of difference in the relations of the hypoblast between the Anura and Urodela. The mesenteron, at the stage represented in fig. 73, forms a wide cavity lined dorsally by a layer of invaginated hypoblast, and ventrally by the yolk-cells. The hypoblast is continuous laterally and in front with the yolk-cells (figs. 72, 74 and 75). At an earlier stage, when the mesenteron has a less definite form, such a continuity between the true hypoblast and the yolk-cells does not exist at the sides of the cavity. [FIG. 75. TRANSVERSE SECTION THROUGH THE CEPHALIC REGION OF A YOUNG NEWT EMBRYO. (After Scott and Osborn.) _In.hy._ invaginated hypoblast, the dorsal part of which will form the notochord; _ep._ epiblast of neural plate; _sp._ splanchnopleure; _al._ alimentary tract; _yk._ and _Y.hy._ yolk-cells.]