152 WALTER HE APE. 



out its length ; at the posterior end it remains thickened, and 

 by the ingrowth of the lateral portions the axial cells first 

 form an arch and then a complete tube, which is the neuren- 

 teric canal and which communicates dorsally with the exterior 

 and ventrally with the yolk-sac. 



This tube is the homologue of the median dorsal diverticulum 

 of the alimentary tract in Amphioxus, i. e. the structure which 

 gives rise to the notochord of that animal, and it is noteworthy 

 that in the Mole it disappears almost entirely before the noto- 

 chord is formed. 



The single layer of cells to which the greater part of the 

 axial hypoblast is reduced at the close of Stage d (No. 8) again 

 increases in bulk during Stages e to J, and gives rise to the 

 notochord. 



As was the case with the lateral hypoblast, the flattening 

 of these cells and their increase in bulk appears to be due, first 

 to the stretching efi'ect of the rapidly deepening medullary 

 groove, and secondly to the release from that strain caused by 

 the depression of the lateral portions of the embryo. 



The isolation of the notochord first occurs in the region of 

 the first protovertebra during Stage a, and extends anteriorly 

 and posteriorly during Stages h and j. 



The isolation is caused by the ingrowth of the lateral hypo- 

 blast below the axial cells, and the latter are isolated either 

 as a solid band or rod, although a lumen may here and 

 there appear in it afterwards. 



At the close of Stage j the notochord is completely separated 

 from the hypoblast, except at two points, viz. at the anterior 

 end, where it is connected with the hypoblast and epiblast, 

 where these two layers fuse to form the mouth, and posteriorly 

 where it is joined to both epiblast, hypoblast, and mesoblast, 

 at the front end of the primitive streak (figs. 49 and 50). 



The origin of the notochord and the manner of its isolation 

 appear to be sufficient reason to regard it as entirely homolo- 

 gous with the notochord of Amphioxus. 



For a review of other opinions on this point I would refer to 

 a discussion in my former paper (No. 8). 



