MA uM MA LI A. 



187 



rated 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. Tiie formation of the 

 notochord takes place from be- 

 fore backwards ; and at the 

 hinder end of tlie 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 hy- 

 poblast in the earlier stage. 



In comparison with Lacerta 

 (|). 1 68 & 1 69) it is obvious that the 

 axial hypoblast and the notochord 

 derived from it have exactly the 

 same relations in Mammalia and 

 Lacertilia. In both they are con- 

 tinued at the hind end of the 

 embryo into the epiblast ; and 

 close to where they join it, the 

 mesoblast and epiblast fuse to- 

 gether to form the primitive 

 streak. The difference between 

 the two types consists in the fact 

 that in Reptilia there is formed 

 a passage connecting the neural 

 and alimentary canals, the front 

 wall of which is constituted by 

 tlie cells wliich fonn the above 

 junction between the notochord 

 and epiblast; and that in Mam- 

 malia this passage — which is only 

 a rudimentary structure in Rep- 

 tilia — has either been overlooked or else is absent. In any case the axial 

 junction of the epiblast and hypoblast in Mammalia is shewn by the above 

 comparison with Lacertilia to represent the dorsal lip of the true verte- 

 brate blastopore. The presence of this blastopore seems to render it clear 

 that the blasto})ore discovered by Ed. van Beneden cannot have the meaning 

 he assigned to it in comparing it with the blastopore of the frog, 



Kblliker adduces tlie fact that the notochord is continuous with the 

 axial cells of the primitive stieak as an argument against its hypoblastic 

 origin. The above comparison with Lacertilia altogether deprives this argu- 

 ment of any force. 



At the stage we have now reached the three layers are definitely 



Fig. 143. A sekies of transverse sec- 

 tions THROUGH THE JUNCTION OF THE PRIMITIVE 

 STREAK AND MP^DUI.LARY GROOVE OF A YOUNG 



GurNEA-piG. (After Schafer.) 



A. is the posterior section. 

 e. epiblast; m. mesoblast; /(. hypoblast; 

 ae. axial epiblast of the primitive streak; 

 (ih. axial hypoblast attached in B. and C. to 

 the epiblast at the rudimentary blastopore ; 

 Tifj. medullary groove; /. rudimentary blas- 

 topore. 



