THE DEVELOPMENT OP THE MOLE. 137 



same manner, although not with precisely the same result, as 

 when the neurenteric canal was formed in Stage d. 



The process in the latter stage involved the ingrowth of the 

 lateral portions of hypoblast and the conversion of the axial 

 portion, containing the neurenteric canal, first into an arc and 

 then into a complete tube. Now the lateral hypoblast grows 

 inwards below the axial portion of primitive hypoblast and 

 unites to form a continuous layer, merely causing the isolation 

 of the axial portion as either a solid rod or band of cells which 

 lies freely between the hypoblast and the medullary canal. It 

 is, however, true that a lumen may appear in some of the por- 

 tions of the notochord which are rod like, although its conver- 

 sion thus into a tube is, so far as I can determine, a secondary 

 matter, and is not connected with the method of isolation. 



The isolation of the notochord first occurs in the region of 

 the first protovertebra during Stage g, and extends during 

 Stages H and j anteriorly and posteriorly. The separation 

 does not, however, appear to be a continuous process, and the 

 shape of the isolated notochord is very various. To demon- 

 strate these facts I have figured several sections of an embryo 

 with nine protovertebrtE (Stage h, figs. 24 and 36 to 42). 



In this embryo, in front of the first protovertebra, the noto- 

 chord is isolated for some distance as a rod or thickened band 

 (figs. 24 and 37), in which a lumen may occasionally be seen 

 (fig. St) : compare also figs. 23 and 25, which are drawings of 

 sections through another embryo of this stage). 



In the region of the first protovertebra, it is in the form of 

 a flattened band consisting of a single row of cells (fig. 38), 

 and this condition persists, except here and there, where the 

 notochord is not completely isolated (fig. 39), until the fourth 

 protovertebra is reached ; here it increases in size. From this 

 point it is more frequently attached to the hypoblast (fig. 40), 

 and posterior to the seventh protovertebra is not isolated at all. 

 Immediately behind the seventh protovertebra it is in the form 

 of an arc (fig. 41), which further backwards flattens out, and 

 the mass, increasing in size, joins the front end of the primitive 

 streak (fig. 42) . 



