NOTOCHOBD 



49 



saw that, in the early stages, it lies under the neural groove (fig. 64), is directly 

 continuous on each side with the primitive entoderm, and at the points where it 

 joins with that layer also with the lateral sheets of mesoderm. By a process of 

 differentiation from before backwards, pari passu with the axial growth, the plate 

 now loses its connexion with the mesoderm-plates, although it continues to pass 

 directly into the lateral entoderm (fig. 76, III.). It next becomes converted 

 into a rounded rod of cells, at first continuous with, then detached from, an under- 

 lying layer of entoderm. The mechanism of this process is probably the doubling 

 up of the notochordal plate and the fusion of the lips of the groove thus formed in 

 the mid-axial line, just as in the case of the neural canal. The rod of cells thus 

 formed is the notochord (figs. 81 and 82). The anterior end of the notochord does 

 not reach to the anterior end of the embryo, but terminates in a recurved 

 point against the wall of the hypophysis cerebri (epithelial part of the pituitary 

 body) in the situation of the future body of the sphenoid bone, and close to the 

 dorsal attachment of the bucco-pharyngeal membrane '(see Development of the 

 Mouth). It will be seen, there- 

 fore, that a portion of the neural 

 canal is prechordal. 



It would seem from the data given 

 for Tarsius by Hubrecht, and also for 

 the dog by Bonnet, that the anterior 

 or head end of the notochord is 

 formed by differentiation directly out 

 of the primitive entodermic plate 

 (fig. 78, p. 54) (protochordal plate, 

 Hubrecht; Ergcinzungsplatte, Bonnet). 



The notochord is essentially an 

 embryonic structure in mammals, 

 although it does not completely dis- 

 appear, for traces of it are to be 

 found throughout life in the middle of 

 the intervertebral discs. When fully 

 developed it is a cylindrical rod com- 

 posed of clear epithelium-like cells, 

 enclosed within a special sheath of 

 homogeneous substance. These cells, 

 although they may become consider 

 ably enlarged and vacuolated, undergo 

 no marked histogenetic change and 

 take no part in the formation of any 

 tissue of the adult. 



amnion 



neural groove 



neurenteric canal 



primitive streak = 



abdominal stall' , -j. 



FIG. 72. SURFACE VIEW OF EARLY HUMAN EMBRYO, 2 MM. 

 IN LENGTH. (After Graf v. Spee, from Kollmann's 

 Entwickelungsgeschichte.) x 80 diameters. 



The amnion is opened, and on the blastoderm are seen 

 the primitive streak, the dorsal opening of the neurenteric 

 canal, and the neural groove. 



Later history of the 

 mesoderm : formation of 

 the mesodermic or primi- 

 tive segments and of the 



coelonic At the time when the neural groove is beginning to appear (figs. 73 

 and 76) a solid sheet of mesoderm extends outwards from the notochordal 

 plate between ectoderm and entoderm, to be continuous outside the embryonic 

 shield with the two layers of the extra-embryonic mesoderm. As the neural folds 

 rise, the central portions of these sheets expand to occupy the spaces, triangular 

 in section (fig. 73), which the folds enclose. These longitudinal thickenings 

 gradually thin off laterally into what is known as the lateral mesoderm (fig. 73). 

 They give origin to the voluntary muscular tissue of the body, and form what 

 may be termed the paraxial, as distinguished from the lateral mesoderm. These 

 paraxial thickenings now become cut up by the occurrence at regular intervals, 



VOL. i. E 



