NOTOCHOBDAL CANAL 



39 



which it is continually being added to. The wings of mesoderm springing from 

 the sides of the notochord-plate are of course continuous with those arising 

 from the sides of the primitive streak. 1 As the axis extends, the streak mesoderm 

 may be conceived as continually becoming converted into axial mesoderm. 



Through this thickened head of the streak a fissure has meanwhile appeared, 

 which becomes converted into a short canal directed obliquely forwards, and 

 opening into the cavity of the yolk-sac (figs. 61, 62, 63, ' 65). It .is called the 

 notochordal or neurenteric canal. In apes (figs. 62 and 63) the canal is considerably 

 wider than in Tarsius, and it is a very prominent feature in early human embryos 

 (figs. 65, 72). In some lower mammals the rabbit, for instance the canal does 

 not break through at any stage, although it is well seen in others for example, in 

 the mole (fig. 64). In the higher Primates the canal is present at a very early 

 stage considerably earlier than in Tarsius ; and it may also be mentioned here 

 that the primitive streak seems to be comparatively short in all the Primates 

 (figs. 62, 63, 65). The canal tunnels through the mass of cells in which the 



FIG. 57. MEDIAN LONGITUDINAL SECTION THROUGH A BLASTODERM OF TARSIUS AT A STAGE ABOUT 

 THE SAME AS REPRESENTED IN FIG. 56, a. (After Hubrecht.) 



p.k., Hensen's (protochordal) knot; emb. ect., embryonic ectoderm; T/.S., yolk-sac; pp, primitive 

 entodermic (protochordal) plate : c.s., ventral mesoderm (connecting stalk). 



notochord-plate ends, and it has been given the name ' notochordal canal ' 

 because of its relations to that plate and the notochord which is formed from it. 

 The term ' neurenteric canal ' signifies properly only the persisting posterior portion 

 of the notochordal canal, this name being given because it corresponds, when the 

 neural groove is formed, to the neurenteric canal of lower vertebrates. It has 

 been already explained that at an earlier stage the protochordal process becomes 

 fused with the entoderm on its under aspect, and that the column of cells arising 

 from the process opens out into a plate, and thus comes to roof -in the cavity 

 under the shield along the middle line. The final result of this process, 

 therefore, is the same as it would have been had the notochordal 

 canal tunnelled the whole length of the protochordal column of 

 cells, and had then broken through into the cavity of the yolk-sac 



1 Hubrecht leaves it open whether these lateral sheets are formed in part or in whole from the 

 protochordal process. Bonnet, in the dog, represents the facts as described in the text. The point is 

 important theoretically, but not from a mere descriptive point of view. 



