134 WALTER HEAPE. 



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

 by the ingrowth of the lateral portions the axial cells fi: 

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

 teric canal and which communicates dorsally with the exteri 

 and ventrally with the yolk-sac. 



This tube is the homologue of the median dorsal diverticuk 

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

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

 that in the Mole it disappears almost entirely before the noi 

 chord is formed. 



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

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

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

 notochord. 



As was the case with the lateral hypoblast, the flattenii 

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

 to the stretching effect of the rapidly deepening medulla 

 groove, and secondly to the release from that strain caused 

 the depression of the lateral portions of the embryo. 



The isolation of the notochord first occurs in the region 

 the first protovertebra during Stage o, and extends anterioi 

 and posteriorly during Stages h and J. 



The isolation is caused by the ingrowth of the lateral hyp 

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

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

 there appear in it afterwards. 



At the close of Stage j the notochord is completely separat 

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

 end, where it is connected with the hypoblast and epiblai 

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

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

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



The origin of the notochord and the manner of its isolati( 

 appear to be sufficient reason to regard it as entirely homol 

 gous with the notochord of Amphioxus. 



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

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



