THE LATER DEVELOPMENT OF THE NOTOCHOED IN 



MAMMALS. 



LEONARD W. WILLIAMS. 



Fro7n the Laboratory of Comparative Anatomy, Harvard Medical School. 



The fate of the notochord in mammals has received, in recent years, 

 very scant attention. This is well exemplified both by the briefness 

 of the discussions of the subject in recent text-books and also in refer- 

 ence works, and by the contradictory statements found in different, or 

 even in a single book. 



The following shades of opinion are found in volumes which have 

 appeared very recently: 



"The notochord here" (in mammals) '^persists longer intervertebrally 

 than intravertebrally, but it disappears entirely by the time the adult 

 condition is reached." (Wiedersheim's Comparative Anatomy, fith 

 German edition, 190G, p. 62, and 3d English edition, 1907, p. GO.) 



"They (the intervertebral discs) are developed, like the bodies around 

 the notochord, persisting parts of this structure forming a central core 

 to each disc." (Piersol's Anatomy, 1907, p. 132.) 



"The notochordal remains lying between each pair of vertebrae with 

 the perichordal tissue grow and remain throughout life as the nuclei 

 pulposi of the intervertebral discs." (Bonnet, Ent^vicklungsgesehichte. 

 1907, p. 381.) 



"The notochord is essentially an embryonic structure in mammals, 

 although it does not completely disappear, for traces of it are to be 

 found throughout life in the middle of the intervertebral discs. Wlien 

 fully developed it is a cylindrical rod composed of clear epithelium-like 

 cells, enclosed within a special sheath of homogeneous substance. These 

 cells, although they may become considerably enlarged and vacuolated, 

 undergo no marked histogenetic change and take no part in the formation 

 of any tissue of the adult. . . . Within the (intervertebral) disc the 

 notochord is enlarged and afterwards converted in each, along with 



The American Jouexal of Anatomy. — -Vol. VIII, No. 3. 



