Develojiment of the ISTotocliord 273 



entiation: the blastemal stage, in which the mesenchymal tissue is 

 loosening up; the precartilaginous, in which a rapid multiplication 

 of cells occurs ; the cartilaginous and the osseous stage. 



In the adult opossum the nucleus pulposus forms a large part of the 

 intervertebral disc. In the tail, it forms 63 per cent of the disc; the 

 remainder being formed, as would be inferred from the great size of 

 its cartilaginous portion in the embryo, almost entirely of fibro-carti- 

 lage. The notochordal tissue has much the same character as in the pig, 

 but the cells have ordinarily but one nucleus and the vacuoles are 

 smaller and of very variable number. The cell or tissue clusters are 

 much larger than in the pig. 



The notochord of an adult mouse forms about 62 per cent of the cer- 

 vical intervertebral discs. Of the remainder, the portion below the 

 nucleus pulposus is of about twice the diameter of the part above it. 

 Fibro-cartilage forms about two-thirds of the former, and but one- 

 third of the latter. The notochordal tissue forms practically a single 

 mass which contains very little mucin, but is surrounded by a thick 

 layer of it, the inner sheath. The cells have usually one nucleus and 

 relatively small vacuoles. 



The structure of the nucleus pulposus of the guinea-pig is quite dif- 

 ferent from that of the mouse. Its relatively small and nearly spheri- 

 cal cells form small strands or clusters that are suspended in a large 

 mass of mucin. They commonly contain small vacuoles. The carti- 

 laginous portion of the disc, as in the pig, is very small. In a few 

 jjlaces the notochord interdigitates with the fibro-cartilage. 



In the adult dog (of advanced but unknown age) the annulus fihro- 

 sus encloses a mass of soft friable tissue of yellowish color which ap- 

 pears, at first sight, to be homogeneous. A small rounded but irregular 

 mass, however, forms the center of the disc. This is quite distinct, 

 and can be lifted out whole, leaving a cavity of sharp and smooth con- 

 tour. Sections of the soft center of the disc at first seem to be com- 

 posed of a single tissue, but it is seen that the cell clusters at the center 

 of the disc are of much more variable size than those of the peripheral 

 part and also that they are of different composition. The peripheral 

 portion of the center of the disc is clearly cartilage containing im- 

 mense cell nests. Not having been able to make out fibrils in the 

 matrix, I am inclined to believe that this is hyaline cartilage with a very 

 soft matrix. Luschka figures and describes, in the human adult, 

 papillge of fibro-cartilage which project into the notochordal tissue and 



