152 THE FOURTH DAY. [CHAP. 



vertebra, consisting as it does of a body or mass investing the 

 notochord, from which springs an arch covering in the neural 

 canal. 



Both body and arch consist at this epoch of but slightly 

 differentiated mesoblast, and the arch springs, to a certain 

 extent, not oily from the posterior segment of the protover- 

 tebra, but also from the anterior or ganglionic segment : 

 though, as seen in Fig. 47, it is far less conspicuous at the 

 level of the latter than of the former. Both neural canal 

 and notochord are thus furnished from neck to tail with a 

 complete investment of mesoblast, still marked, however, by 

 the transparent lines indicating the fore and aft limits of 

 the several protovertebrse. This is sometimes spoken of as 

 the " membranous " vertebral column. 



The ganglionic rudiment, placed anteriorly to its corre- 

 sponding primary vertebra, consists in chief of a large oval 

 swelling, the ganglion of the posterior root (Fig. 47, pr). 

 At a little distance beyond its ganglion, the posterior root is 

 joined by the anterior root (ar) ; and the two form together 

 the common nerve-trunk, which is at first very short. Com- 

 pared with either root or with the nerve-trunk the ganglion, 

 at this epoch and for some time afterwards, seems dispropor- 

 tionably large. At first, neither root is connected with the 

 involuted epiblast of the neural canal. Very speedily, how- 

 ever, they both come to be united with that portion of the 

 neural tube which, as we shall presently state, gives origin 

 to the grey matter of the spinal cord. It is, however, easier 

 to trace the fibres of the anterior root into the cord, than 

 those of the posterior, and they can be followed in it for a 

 greater distance. 



On the fourth day the nerves are composed of cells whose protoplasm is 

 beginning to become converted into fibres. Amongst these fibres, the nuclei 

 of the cells with distinct nucleoli are thickly scattered. On the sixth day and 

 still more on the seventh the fibrillated structure of the nerves is much more 

 distinct and the nuclei are far less numerous. 



The ganglia on the fourth day are composed of numerous nuclei surrounded 

 by protoplasm, between which the fibres of the nerves pass. Covering this mass 

 of nucleated cells is a layer of mesoblast (also derived from the tissue of the 

 protovertebra) which, by the sixth day, forms a kind of sheath around them. 



The cells of the ganglia from the fourth to the sixth day contain round 

 granular nuclei with distinct nucleoli very similar to the nuclei of the 

 ordinary mesoblast-cells. The limits of the protoplasm of the individual cells 

 are as a general rule not easily seen, but with care may be made out. The 

 amount of protoplasm round each nucleus appears however to be very small. 



