ON THE NOTOCHORD 247 



down of the fundamental structures of the future systemic 

 nervous system. 



In this earliest skeletal development, we see in full 

 activity, the membranous, and cartilaginous, methods, 

 of ossification (Fig. 105), in order to overtake the much 

 required developmental work of providing, at the earliest 

 period, both external and internal means, of organic 

 support and protection, before the usual factors for 

 permanent osseous structures have been laid down, or 

 come into existence, and it is most remarkable and 

 suggestive to find that these phenomena are displayed 

 in connection with the nascent systemic nerve centres, 

 where the basi-sphenoid cartilages are developing, round, 



FIG. 106. WHITE FIBRO-CARTILAGE FROM AN INTERVERTEBRAL DISK 

 (HUMAN). Highly magnified. (E. A. S.) 



The concentric lines around the cells indicate the limits of deposit of successive cap- 

 sules. One of the cells has a forked process which extends beyond the hyaline 

 area surrounding the cell, amongst the fibres of the general matrix. 



and under, the central brain structures, and the vertebral 

 bodies, under, and in front of, the developing spinal 

 cord. 



The notochord may, thus, be compared to an internal 

 periosteum, and, therefore, we may correctly call it the 

 end-osteum of the vertebral column, as it traverses its 

 united textural units, carrying their earthy nutritive 

 elements, as long as it is necessary for it to do so, and 

 disappearing, into anatomical obscurity, on the establish- 

 ment of other, and permanent, means, of carrying on, 

 its hitherto essential, vehicular work. Thus employed, 

 it bridges over a period in the process of skeletal growth, 

 when the periosteal structures of the skeleton, generally, 

 are in course of evolution, and, of course, before the 

 period at which protection and support are absolutely 

 necessary for securing the growth, and evolution, of the 

 future systemic nerve organs, the brain, and spinal cord. 



