182 



MANUAL OF HISTOLOGY. 



that is, of concentric portions of fibrous tissue crossing each other per- 

 pendicularly and obliquely. This ring has sometimes the character of 

 simple connective-tissue, sometimes that of an elastic or fibro-cartilage. 

 The central portion is soft, and often possesses a cavity containing a jelly- 

 like mass, the so-called gelatinous or pulp nucleus (a). This is made up 

 in the adult of ragged processes of the peripheral fibrous cartilage, which 

 lie closely crowded together, leaving in their midst a cavity filled with 

 gelatinous matter. 



While the pulp nucleus resembles more arid more the fibrous ring as 

 its solidity increases at an advanced period of life, it shows a very dif- 

 ferent bearing in the infant and foetus. Here (fig. 173), at an earlier 

 embryonic epoch, the origin of the nucleus is quite manifest. It takes 

 its rise, namely, from the exuberant growth of the residue of a rudimentary 

 foetal structure, the chorda dorsalis (Luschka). This latter, which per- 

 sists among the lowest animals for a part of, or the whole of, life, pre- 

 sents itself as a cylindrical rod blunted before and fining off to a point 

 posteriorly. It passes from the base of the skull down to the lower end 

 of the vertebral column in the place of the bodies of the bones making 

 up the latter. 



It consists of a tissue which may be perhaps classed among the 

 cartilages, formed of transparent cells crowded close together, and 

 enclosed in a homogenous envelope. The chorda dorsalis disappears 

 almost entirely with the formation of the cartilaginous base of the skull 

 and vertebral column. However, in the intervertebral ligaments there 



remains a space filled with the 

 characteristic cells of the notochord 

 (d), which may even extend into 

 the body of the vertebra themselves. 

 Thus we see it in the embryo of 

 ten weeks old. 



In the foetus in the fifth month 

 (fig. 174) we find here again very 

 similar cells, with a single vesicular 

 nucleus, and about '01 36-0 '01 80 

 mm. in diameter. From these (1, a) 

 there are gradations up to some of 

 0*0413 mm! and upwards, in which 

 we encounter double and quad- 

 rupled nuclei, or even still more(&&), 

 or the same number of endogenous 

 cells (c, d). Beside these, spring- 

 ing from the further growth of such 

 parent-cells very large bodies are 

 met with, measuring up to 0-1128 

 mm., of tough constitution and 

 transparent appearance. They are 

 partly filled with distinct daughter- 

 cells, but principally with extreme- 

 ly numerous transparent globules 

 of a metamorphosed albuminous 

 substance. 



In the still infantile body we meet with the same structures with a 

 tough envelope (the thickened parent capsule), which may attain a 



Fig. 174. Descendants of the cells of the chorda 

 dorsalis in a foetus of five months old, and in an 

 infant. 1, Cells from the foetus of five months; 

 2, single cell from the infant; 3, another, with 

 daughter-cells; 4 and 5, a structure which has 

 taken its rise from a very much enlarged parent- 

 cell, with nucleated cells and many transparent 

 globules of albuminous nature. 



