38 HISTOLOGY 



lary tube. Thus the entire nervous system, central and peripheral, is 

 ectodermal in origin. 



Entoderm. Before considering the chief derivatives of the entoderm, 

 the notochord (or chorda dorsalis) may be briefly described. In the 

 lowest vertebrates it is an important supporting structure, and is regarded 

 as "the primitive forerunner of the vertebral column." It arises in 

 young mammalian embryos as a median longitudinal band of cells in 

 the entodermal layer, immediately below the floor of the medullary groove. 

 In the diagram, Fig. 25, H, it is shown as an elevation; in Fig. 29, it ap- 

 pears as a group of cells completely detached from the underlying ento- 

 derm. It then forms a longitudinal rod extending forward from the primi- 

 tive ^knot to the under side of the brain, as seen in the longitudinal 

 section of the chick embryo, Fig. 28. Later it becomes surrounded by 

 niesodermal cells, which develop into the bodies (or centra) of the verte- 

 brae together with the intervertebral ligaments between them. These are 



-iNch. 



Nch. 



Nch, 



FIG. 26. THE NOTOCHORD. 

 A, in a sheep embryo of 14.6 mm. (after Minot); B, in a cod fish; C, in man (after Dwight). 



shown in Fig. 26, A, as alternating light and dark areas respectively. The 

 notochord in passing through them shows "segmental flexures" (Minot). 

 In the vertebral column of a fish (Fig. 26, B) the central notochordal 

 rod has expanded between the bodies of the vertebras so as to form large 

 lenticular masses of gelatinous pulp. These retain a very slender con- 

 nection with one another. In the human adult, the notochord is repre- 

 sented by the series of detached expansions, or nuclei pulposi, one of which 

 occurs in each intervertebral ligament (Fig. 26, C). These nuclei are 

 composed of a peculiar tissue, the development of which has been de- 

 scribed by L. W. Williams (Amer. Journ. Anat., 1908, vol. 8, pp. 251- 

 284). The notochord is very rarely the source of tumors. Occasionally, 

 owing to its connection with the entoderm, which is retained longest 

 anteriorly, it gives rise to a pharyngeal recess (Huber, Anat. Record, 

 1912, vol. 6, pp. 373-404). 



