X.] THE NOTOCHORD. 325 



are united together. The mesoblast plates at the two 

 sides remain in the meantime quite free. Slightly 

 further back, but before the primitive groove is reached, 

 the epiblast and hypoblast are connected together by a 

 cord of cells (Fig. 105 B,/), which in the section next 

 following becomes detached from the hypoblast and 

 forms a solid keel projecting from the epiblast. In the 

 following section the hitherto independent mesoblast 

 plates become united with this keel (Fig. 105 A) ; and 

 in the posterior sections, through the part of the primi- 

 tive streak with the primitive groove, the epiblast and 

 mesoblast continue to be united in the axial line, but 

 the hypoblast remains distinct. These peculiar relations 

 may shortly be described by saying that in the axial 

 line 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. 



The notochord. The thickened axial portion of the 

 hypoblast in the region of the embryo becomes sepa- 

 rated, as we have already pointed out, from the lateral 

 parts 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 it is continued 

 into the mass of cells which forms the axis of the primi- 

 tive streak, becoming therefore at this point continuous 



