2GS Leonard W. Williams 



of notochordal tissue is about six times as great as its thickness. The 

 peripheral portion of the notochordal syncytium is more continuous 

 and regular, and is also denser. The notochordal tissue (Fig. 15) con- 

 tains a few very large vacuoles at its center, but elsewhere is filled with 

 a multitude of small vacuoles. 



The vast increase in nuclei which accompanies the growth of the 

 notochord is apparently due entirely to mitotic division, for in well- 

 preserved material mitotic figures are abundant and there is no sugges- 

 tion of amitotic division. In poorly preserved tissue mitosis cannot be 

 recognized, and the irregularity of certain nuclei suggests amitosis, but 

 this condition is probably due entirely to improper fixation. 



In a larger embryo (250 mm.) the large central vacuoles of the 

 notochord are apparently moving toward the periphery, and in a few 

 places they have broken through the dense peripheral layer into the 

 inner sheath. As they reach the surface these vacuoles often tear off 

 portions of the dense peripheral layer which form rounded isolated 

 masses. I find upon the lower edge of a single disc in this embryo, an 

 interdigitation of processes of the cartilage and notochord such as 

 Kolliker and Luschka describe in the adult man. The cartilage has 

 constricted off a few small nodules of notochordal tissue which arc 

 assuming the structure characteristic of adult notochordal tissue. Large 

 vacuoles which do not consist of mucin are forming in the cells which 

 have become visible in the syncytium. 



In the half-grown pig the notochord has encroached yet farther upon 

 the remainder of the disc and forms about .74, and the fibrous tissue 

 and fibro-cartilage .2G of the diameter of the disc. In shape the noto- 

 chordal expansion is a very thin, lenticular disc. It is still surrounded 

 by its inner sheath of mucin, which has become more dense, and after 

 fixation in Zenker's solution it appears very finely and somewhat irregu- 

 larly fibrillar. It now stains with hsematoxylin more strongly than the 

 fibro-cartilage. The formerly continuous peripheral sheet of dense syn- 

 cytial tissue is now broken in many places by large masses of mucin, 

 and in other regions it contains large vacuoles which seem about to 

 escape into the inner sheath. The formation of mucin has continued 

 until the center of the notochordal mass consists of a large quantity 

 of mucin in which the slender syncytial network seems suspended. Be- 

 tween the very loose central mass of the notochord and the much broken 

 dense peripheral layer, there is a zone which contains moderately small 

 vacuoles in a sjmcytial mass. The mucin is gradually replacing a large 



