270 Leonard W. Williams 



for they are not stained by osmic acid, Sharlach E, or Sudan III. 

 The cytoplasm of these cells contains granules of glycogen. 



It is a cause of regret that I have not been able to obtain notochordal 

 tissue from immature pigs of various ages' and from very old animals 

 in order to follow carefully the process of formation of these cells and 

 the ultimate modifications of the nucleus pulposus. It should be noted 

 that although there are several points of similarity between notochordal 

 tissue and cartilage in the pig and, as we shall find later, greater simi- 

 larity in other animals, nevertheless, they are distinct tissues. The 

 nucleus pulposus is formed entirely by the notochord. 



Kolliker describes and figures clusters of cells from a child of one year 

 which are essentially like the cell clusters of the adult pig; and Fric 

 describes and pictures the same tissue from the nucleus pulposus of the 

 human adult. Both men, however, consider that this notochordal tissue 

 forms only the central part of the nucleus pulposus and that the weak 

 fibro-cartilage of the disc is the peripheral portion of the nucleus pul- 

 posus. No transitions occur between the two tissues, and the inclusion 

 of the fibro-cartilage in the nucleus pulposus is merely a matter of 

 definition, not a question of fact; but unfortunately the description of 

 the nucleus pulposus as formed of notochord and cartilage has led many 

 to believe that it is produced by a fusion of the two tissues. The two 

 tissues remain as distinct in the adult, despite their interlocking pap- 

 illse, as in the new-born child, in which they are separated by a sharp 

 boundary. Both Kolliker and Fric call the notochordal tissue of the 

 nucleus pulposus a "remnant" (Rest) of the notochord. This termin- 

 ology is not allowable because it makes the remnant of a part greater 

 than the whole; for the notochordal tissue of a single disc is much 

 greater than the entire notochordal rod of the embryo. 



The Notochord in Other Mammals. 



It has not been possible to follow out the development of the noto- 

 chord in other mammals as carefully as in the pig, nor is it necessary, 

 for unless there appears in the literature of the subject or in the tissues 

 of the adult some confusion or doubtful evidence, it is permissible to 

 assume that similar structures in mammals have a similar developmen- 

 tal history. 



The notochord of an opossum embryo of 7.5 mm. is a slender rod 

 without segmental undulations. Anteriorly it is surrounded by an 



