[FIG. 100. TRANSVERSE SECTION THROUGH THE EMBRYONIC REGION OF THE BLASTODERM OF A CHICK AT THE TIME OF THE FORMATION OF THE NOTOCHORD, BUT BEFORE THE APPEARANCE OF THE MEDULLARY GROOVE. _ep._ epiblast; _hy._ hypoblast; _ch._ notochord; _me._ mesoblast; _n._ nuclei of the germinal wall _yk._ yolk.] As regards the mesoblast of the primitive streak, in a purely objective description like that given above, the greater part of it may fairly be described as being derived from the epiblast. But if it is granted that the primitive streak corresponds with the blastopore, it is obvious to the comparative embryologist that the mesoblast derived from it really originates from the lips of the blastopore, as in so many other cases; and that to describe it, without explanation, as arising from the epiblast, would give an erroneous impression of the real nature of the process. [FIG. 101. TRANSVERSE SECTION OF A BLASTODERM INCUBATED FOR 18 HOURS. The section passes through the medullary groove _mc._, at some distance behind its front end. A. epiblast. B. mesoblast. C. hypoblast. _m.c._ medullary groove; _m.f._ medullary fold; _ch._ notochord.] The differentiation of the embryo may be said to commence with the formation of the notochord and the lateral plates of mesoblast. Very shortly after the formation of these structures the axial part of the epiblast, above the notochord and in front of the primitive streak, which is somewhat thicker than the lateral parts, becomes differentiated into a distinct medullary plate, the sides of which form two folds--the medullary folds--enclosing between them a medullary groove (fig. 101). In front the two medullary folds meet, while posteriorly they thin out and envelop between them the front end of the primitive streak. On the formation of the medullary folds the embryo assumes a form not unlike that of the embryos of many Ichthyopsida at a corresponding stage. The appearance of the embryo, and its relation to the surrounding parts is somewhat diagrammatically represented in fig. 102. The primitive streak now ends with an anterior swelling (not represented in the figure), and is usually somewhat unsymmetrical. In most cases its axis is more nearly continuous with the left, or sometimes the right, medullary fold than with the medullary groove. In sections its front end appears as a ridge on one side or on the middle of the floor of the widened end of the medullary groove. [FIG. 102. SURFACE VIEW OF THE PELLUCID AREA OF A BLASTODERM OF 18 HOURS. None of the opaque area is shewn, the pear-shaped outline indicating the limits of the pellucid area. At the hinder part of the area is seen the primitive groove _pr._, with its nearly parallel walls, fading away behind, but curving round and meeting in front so as to form a distinct anterior termination to the groove, about halfway up the pellucid area.