Development of the N'otochord 261 



they persist even in enibr3'os of 14 mm. Only a small part of the neu- 

 ral and costal processes has been transformed into precartilage. At 

 the level of the base of the spinal cord, the neural process of the 

 scleromere is approximately triangular in frontal section. Two acute 

 and equal angles are directed inward and outward and its obtuse angle 

 is directed forward. The last lies close behind the spinal nerve and is 

 continuous with the "inter dorsal" and the "interdiscal ligaments" (of 

 Bardeen) which lie respectively upon the outer and inner sides of the 

 nerve. The outer angle projects between the myotomic muscles and 

 is separated from the blastemal tissue of the anterior end of the inter- 

 dorsal and interdiscal ligaments of the next segment by the ramus 

 dorsalis of the spinal nerve. Above the ramus dorsal is there is a 

 rounded blastemal mass which cannot be assigned to one or the other 

 segment. The outer angle of the neural process remains blastemal for 

 some time and seems finally to form the myoseptum. The inner angle 

 is continuous below and anteriorly with the costal process and with the 

 periphery of the intervertebral disc. The precartilage of the neural 

 arch appears upon the posterior side of the inner angle of the blastemal 

 neural process. It reaches only to the upper edge of the myotomes. 

 The costal process is largely blastemal, but it contains the small elon- 

 gated precartilage of the rib. 



In embryos of 14 mm. (Fig. 4) the vertebrae are larger and are more 

 definitely outlined. A sheet of elongated, closely-placed nuclei, formed 

 by the extension of the interdiscal ligament and by its fusion with the 

 perichordal septum, surrounds the vertebra and binds together the suc- 

 cessive intervertebral discs. It represents the fibrous tissue of the 

 perichondrium and of the dorsal and ventral common ligaments. A 

 mass of dense blastemal tissue, which is perforated by the ramus dor- 

 salis, extends from the neural arch to the rib. 



The vacuolization of the notochord has continued and an inner sheath, 

 which is much thicker than the outer sheath, has been formed. The 

 inner sheath and the vacuoles of the notochord are composed of mucin 

 or a mucin-like substance, for they are stained by mucicarmine. For 

 convenience this substance is referred to hereafter in this paper as 

 mucin, but I do not intend to convey the impression that its compo- 

 sition is even approximately known. The notochord is surrounded 

 within its inner sheath by an apparently continuous wall which is 

 formed by the exposed walls of its superficial cells. 



The intervertebral disc begins a little later, in a pig of 14.8 mm., 

 to differentiate into a looser central portion, with nuclei irregular both 



