32 THE NOTOCHORD. 



produce the groove which is enclosed between them. The groove which is thus 

 early formed in front of, but not, as was formerly supposed, in continuity with the 

 primitive groove (Dursy), is no less than the rudiment of the whole central nervous 

 svstem, and it is accordingly known as the neural or medullary groove., the folds which 

 bound it being termed the medullary folds. By the time that the neural groove is 

 formed, the mesoblast has generally extended forwards from either side of the 

 primitive streak, burrowing between the epi- and hypoblast, and as the folds become 

 developed, this mesoblast fills up the space below the epiblast, triangular in section, 

 which each fold encloses, so that on either side of the neural groove there is now a 

 longitudinal thickening of mesoblast, entirely separated from its fellow of the 

 opposite side by the meeting of epi- and hypoblast at the bottom of the neural 

 groove (fig. 31), and gradually thinning off laterally into what is known as the 



Fig. 33. MIDDLE OF THE SECTION SHOWN IN FIG. 31, 



MAGNIFIED TO SHOW THE IETAILS OF ITS STRUCTURE. 



(E. A. S.) 



ep, me, hy, m.g.. as above ; ncA, notochordal thickening 

 of median hypoblast. 



lateral plate of mesoblast. These two longi- 

 tudinal thickenings of mesoblast give origin to 

 most of the muscular and skeletal tissues of the 

 body ; they form what may be termed the 

 paraxial as distinguished from the laieral meso- 

 blast. Somewhat later the medullary folds 



become bent over the neural groove, and meet one another in the middle line 

 (fig. 32). Here they blend together, and the groove becomes converted into a 

 canal the neural canal. Of the two layers of epiblast which are formed from 

 the folds, one is now the roof of the canal, the other is the epiblast of the dorsal 

 surface of the embryo. The layers are at first in contact with one another, but 

 subsequently mesoblast passes between them (forming the membrana reuniens 

 superior of Remak). The closure of the neural groove begins in the posterior 

 cephalic region, and thence extends forwards and backwards. 



Notochord. Running along the bottom of the neural groove there may soon 

 be seen, when the blastoderm is viewed from above, a linear shading, which appears 

 to start from the anterior end of the primitive streak and passing forwards becomes 

 gradually lost towards the anterior end of the neural groove. Transverse sections 

 across the latter show that this appearance of shading is due to a longitudinal 

 thickening of the hypoblast along the middle line (fig. 33) ; the central cells of 

 this layer becoming enlarged and gradually separating themselves off to form a rod- 

 like column, which lies between epi- and hypoblast just below the neural groove 

 (fig. 32, ch). When so separated, the column is known as the chorda dorsalis, or 

 notochord, a structure which, along the middle line of the early embryo, replaces the 

 mesoblast, and which is at first, as before said, continuous with the united epi- and 

 hypoblast at the anterior end of the primitive streak. The actual separation of the 

 notochordal cells from the hypoblasfc occurs first a little behind the anterior end of 

 neural groove, and progresses backwards, although the hypoblastic thickening occurs 

 first at the posterior end of the neural groove and is in fact directly continuous with 

 the united column of epiblast and hypoblast which forms the anterior end of the 

 primitive streak. The neurenteric canal passes through the thickened posterior 

 extremity of the notochord, where this is continuous with the anterior end of the 

 primitive streak (see fig. 34). It is continued in mammals a short distance along the 

 notochord as a canal (prolonged forwards into a groove) (fig. 34, 5), which has been 



