Development of the jSTotochord 271 



anhistic sheath, and its tissue is s}Ticytial and vacuolated. At the 

 base of the tail, the notochord is cellular, vacuolated, and the some- 

 what thickened exposed cell-walls form its only sheath. In the tail, 

 vacuoles are absent, and I am unable to find either cell-walls or sheath. 

 A mesenchymal sheath surrounds the notochord in the tail and in the 

 posterior part of the trunk, but there is no indication of a perichordal 

 septum. 



The dense sclerotomic tissue is interrupted, at the level of the noto- 

 chord, by broad intersegmental zones of looser tissue. The myocoele 

 has closed, and I do not find the fissure of von Ebner. The lighter zones 

 are broad anteriorly, and in the neck and thorax they have become 

 precartilage. The intervertebral discs lie in the "third and fourth fifths 

 of the segments, and are consequently a trifle farther back than in the 

 pig. In an embryo of 8 mm., the centers of the discs of the anterior 

 part of the spine have become precartilage. 



The chondrostyle is well formed in an embryo of 11 mm. (Series 

 925), and is a cylindrical rod which encloses the notochord and bears 

 the ribs and neural arches. Near the tip of the tail, the vertebrae are 

 represented by broad zones of loose blastemal tissue which are separated 

 by the denser tissue of the intervertebral discs. In the middle of the tail 

 ..the notochordal undulations have appeared, and the crest of each 

 undulation lies in an intervertebral disc, the trough in the precartilage 

 of the vertebra. The center of each intervertebral disc in the sacral 

 region is now precartilage, and the peripheral portion of each disc 

 forms a slight thickening of the continuous perichondrium of the 

 chondrostyle. The enlargement of the vertebrae, which is caused by 

 their chondrification, is forcing the notochord in the lumbar region 

 from the vertebrae into the intervertebral discs and is also both com- 

 pressing the cells of the disc and carrying the lower and lateral parts 

 of the disc away froui the notochord. This process makes the verte- 

 brae, which before chondrification are smaller than the intervertebral 

 discs, larger than the intervening discs. In the trunk and neck, how- 

 ever, the chondrification of the cartilage of the discs has caused them 

 to become of nearly as great diameter as the vertebra. In this region, 

 therefore, the discs are recognizable only by the notochordal enlargement, 

 by a slight compression of the cartilage cells, and by the slight thicken- 

 ing of the perichondrium which represents the fibrous portion of the 

 disc. The result of all these processes is that the spine of this embryo 

 is represented anteriorly by the nearly cylindrical chondrostyle; in the 



