274 Leonard W. AVilliams 



which contain similar but much smaller nests of cells. The central 

 mass consists of a firm and apparently fibrillar matrix in which are 

 embedded, without any regularity, clusters of cells of various size and 

 appearance. Certain clusters contain small non-vacuolated cells which 

 are from 10 to 14 microns in diameter and stain intensely with Orange 

 G. The nuclei have a diameter about one-third as large as that of 

 the cells and each contains small masses of chromatin and a nucleolus. 

 Many cells contain nuclei, and the cells are often arranged in pairs 

 or fours like cartilage cells. These cells are probably non-vacuolated 

 notochordal cells. Other cell clusters are larger and are enclosed 

 in a definite rounded cavity. Many of the cells in these clusters are of 

 the type just described, the others contain vacuoles of various sizes. 

 The cells with large vacuoles are similar to the notochordal cells de- 

 scribed by Luschka, Kolliker and Fric in man, and are somewhat like 

 those of the pig's notochord. There can be little doubt that these are 

 also notochordal cells. The boundary of the central mass or notochord 

 can be recognized under the microscope by a slight difference in staining 

 property and texture between the matrix of the cartilage and of the 

 notochord. Not having studied the formation of the intervertebral 

 disc of the dog, I am unable to assert more than the probability that 

 its soft center is formed of notochordal and cartilaginous tissue, and 

 that as age advances the two tissues become more and more alike. 



The morphological and physiological meaning of the segmentation 

 of the notochord is cjuite clear. The notochord and its membranous, 

 cartilaginous or bony sheath have been assumed to be developed in 

 inverse ratio to one another : the former being the predominant struct- 

 ure in less, and the latter in more specialized forms. This is, in a 

 general way, true, but the notochord does not degenerate in mammals. 

 On the contrary, while it loses its primary continuity and surrenders 

 to the vertebrae a part of its primary function, nevertheless the noto- 

 chord continues to perform a part of its primary function, but in a 

 somewhat different way and in connection with the segmented spine. 

 The notochord is primarily surrounded by a continuous sheath of con- 

 nective tissue, in which later appear isolated metameric cartilaginous 

 elements. In mammals the cartilaginous elements unite, as we have 

 seen, at such an early stage in development that they may scarcely be 

 said to exist as separate units. The chondrostyle is deeply constricted 

 near the center of each segment by the fibrous tissue of the intervertebral 

 disc. Without these constrictions the chondrostyle would be too rigid 



