Shortly afterwards, at a slightly later stage than that represented in fig. 96, an important change takes place in the constitution of the hypoblast in front of the primitive streak. The rounded cells, of which it is at first composed (fig. 98), break up into (1) a layer formed of a single row of more or less flattened elements below--the hypoblast--and (2) into a layer formed of several rows of stellate elements, between the hypoblast and the epiblast--the mesoblast (fig. 99). A separation between these two layers is at first hardly apparent, and before it has become at all well marked, especially in the median line, an axial opaque line makes its appearance in surface views, continued forwards from the front end of the primitive streak, but stopping short at a semicircular fold--the future head-fold--near the front end of the area pellucida. In section (fig. 100) this opaque line is seen to be due to a special concentration of cells in the form of a cord. This cord is the commencement of the notochord (_ch_). In some instances the commencing notochord remains attached to the hypoblast, while the mesoblast is laterally quite distinct (_vide_ fig. 100), and is therefore formed in the same manner as in most Ichthyopsida; while in other instances, and always apparently in the Goose (Gasser, No. 127), the notochord appears to become differentiated in the already separated layer of mesoblast. In all cases _the notochord and the hypoblast below it unite with the front end of the primitive streak_; with which also the two lateral plates of mesoblast become continuous. [FIG. 99. TRANSVERSE SECTION THROUGH THE EMBRYONIC REGION OF THE BLASTODERM OF A CHICK SHORTLY PRIOR TO THE FORMATION OF THE MEDULLARY GROOVE AND NOTOCHORD. _m._ median line of the section; _ep._ epiblast; _ll._ lower layer cells (primitive hypoblast) not yet completely differentiated into mesoblast and hypoblast; _n._ nuclei of germinal wall.] From what has just been said it is clear that in the region of the embryo the mesoblast originates as two lateral plates split off from the hypoblast, and that the notochord originates as a median plate, simultaneously with the mesoblast, with which it may sometimes be at first continuous. Kölliker holds that the mesoblast of the region of the embryo is derived from a forward growth from the primitive streak. There is no theoretical objection to this view, and I think it would be impossible to shew for certain by sections whether or not there is a growth such as he describes; but such sections as that represented in fig. 99 (and I have series of similar sections from several embryos) appear to me to be conclusive in favour of the view that the mesoblast of the region of the embryo is to a large extent derived from a differentiation of the primitive hypoblast. I am however inclined to believe that some of the mesoblast cells of the embryonic region have the derivation which Kölliker ascribes to all of them.