62 WALTEE llEAi*E. 



Historical. — The arrangement of the layers at the front end 

 of the primitive streak has been described by Hensen (No. 

 12), Schafer (No. 26), and recently by Lieberkiihn (No. 20). 



According to the former, the axial cells below the medullary 

 groove at its posterior end are thickened and join the primitive 

 streak at the node of Hensen, this portion of the primitive 

 streak being composed of epi-, hypo-, and mesoblast fused 

 together. 



Schafer describes a similar arrangement in a somewhat 

 different manner. According to him the axis of the embryo in 

 this region is " occupied by a continuous column of cells, which 

 inseparably connect the epiblast and hypoblast, and, traced 

 from behind forwards, would appear to be chiefly epiblastic in 

 origin." 



This author does not appear to believe that the hypoblast 

 takes part in the formation of the primitive streak, and he 

 therefore considers, I imagine, that the latter organ begins 

 where the hypoblast lies free below the mesoblast. 



Neither of these observers described any canal perforating 

 the blastoderm at this point. 



Balfour, however, in his ' Comparative Embryology ' (No. 3), 

 has expressed his belief that the axial cord of cells described 

 by Schafer is the rudiment of the neurenteric canal of Lacertilia 

 and birds. 



Lieberkiihn agrees with Hensen as to the arrangement of 

 the layers at the front end of the primitive streak, and further 

 finds a canal present in the mesoblast, which grows forwards 

 from the front end of the " node." He states that the canal 

 arises in the mesoblast, and does not open dorsally through the 

 epiblast, but that it is prolonged forwards, and opens below 

 through the hypoblast. 



The notochord he believes to be formed from mesoblast, 

 which secondarily becomes united with the hypoblast. This 

 author also compares the neurenteric canal, such as he finds in 

 mammals, with that of birds and lizards, and declares they are 

 essentially different, inasmuch as in the latter the canal arises 



