the hypoblast becomes _united with the epiblast at the posterior end of the embryo_; and that the cells which connect the hypoblast and epiblast are posteriorly continuous with the fused epiblast and mesoblast of the primitive streak, the hypoblast in the region of the primitive streak having become distinct from the other layers. [FIG. 141. EMBRYONIC AREA OF A SEVEN DAYS' EMBRYO RABBIT. (From Kölliker.) _o._ place of future area vasculosa; _rf._ medullary groove; _pr._ primitive streak; _ag._ embryonic area.] [FIG. 142. TRANSVERSE SECTION THROUGH AN EMBRYO RABBIT OF EIGHT DAYS. _ep._ epiblast; _me._ mesoblast; _hy._ hypoblast; _mg._ medullary groove.] [FIG. 143. A SERIES OF TRANSVERSE SECTIONS THROUGH THE JUNCTION OF THE PRIMITIVE STREAK AND MEDULLARY GROOVE OF A YOUNG GUINEA-PIG. (After Schäfer.) A. is the posterior section. _e._ epiblast; _m._ mesoblast; _h._ hypoblast; _ae._ axial epiblast of the primitive streak; _ah._ axial hypoblast attached in B. and C. to the epiblast at the rudimentary blastopore; _mg._ medullary groove; _f._ rudimentary blastopore.] The peculiar relations just described, which hold also for the rabbit, receive their full explanation by a comparison of the Mammal with the Bird and the Lizard, but before entering into this comparison, it will be well to describe the next stage in the rabbit, which is in many respects very instructive. In this stage the thickened axial portion of the hypoblast in the region of the embryo becomes separated from the lateral part as the notochord. Very shortly after the formation of the notochord, the hypoblast grows in from the two sides, and becomes quite continuous across the middle line. The formation of the notochord takes place from before backwards; and at the hinder end of the embryo the notochord is continued into the mass of cells which forms the axis of the primitive streak, becoming therefore at this point continuous with the epiblast. The notochord in fact behaves exactly as did the axial hypoblast in the earlier stage.