In the course of further growth the area pellucida soon becomes pyriform, the narrower extremity being the posterior. The primitive streak (fig. 96) elongates considerably, so as to occupy about two-thirds of the length of the area pellucida; but its hinder end in many instances does not extend to the posterior border of the area pellucida. The median line of the primitive streak becomes marked by a shallow groove, known as the primitive groove. [FIG. 97. TRANSVERSE SECTION THROUGH THE FRONT END OF THE PRIMITIVE STREAK OF A BLASTODERM OF THE SAME AGE AS FIG. 96. _pv._ primitive groove; _m._ mesoblast; _ep._ epiblast; _hy._ hypoblast; _yh._ yolk of germinal wall.] During these changes in external appearance there grow from the sides of the primitive streak two lateral wings of mesoblast cells, which gradually extend till they reach the sides of the area pellucida (fig. 97). The mesoblast still remains attached to the epiblast along the line of the primitive streak. During this extension many sections through the primitive streak give an impression of the mesoblast being involuted at the lips of a fold, and so support the view above propounded, that the primitive streak is the rudiment of the coalesced lips of the blastopore. The hypoblast below the primitive streak is always quite independent of the mesoblast above, though much more closely attached to it in the median line than at the sides. The part of the mesoblast, which I believe to be derived from the primitive hypoblast, can generally be distinctly traced. In many cases, especially at the front end of the primitive streak, it forms, as in fig. 97, a distinct layer of stellate cells, quite unlike the rounded cells of the mesoblastic involution of the primitive streak. [FIG. 98. LONGITUDINAL SECTION THROUGH THE AXIAL LINE OF THE PRIMITIVE STREAK, AND THE PART OF THE BLASTODERM IN FRONT OF IT, OF AN EMBRYO CHICK SOMEWHAT YOUNGER THAN FIG. 99. _pr.s._ primitive streak; _ep._ epiblast; _hy._ hypoblast of region in front of primitive streak; _n._ nuclei; _yk._ yolk of germinal wall.] In the region in front of the primitive streak, where the first trace of the embryo will shortly appear, the layers at first undergo no important changes, except that the hypoblast becomes somewhat thicker. Soon, however, as shewn in longitudinal section in fig. 98, the hypoblast along the axial line becomes continuous behind with the front end of the primitive streak. Thus at this point, which is the future hind end of the embryo, the mesoblast, the epiblast, and the hypoblast all unite together; just as they do in all the types of Ichthyopsida.