256 Leonard W. Williams 



end of the "horizontal plate" to the intersegmental artery, and the 

 "vertebral arch" which extends outward from that artery between the 

 myotomes. Weiss finds that the transverse portion of the primitive 

 vertebra forms the annulus fihrosus of the intervertebral disc; Schnltze 

 and Bardeen, that it forms part of that structure and also the anterior 

 portion of the vertebra. Schultze asserts that the rib belongs to the 

 posterior half of the segment, and Bardeen that from this point of 

 origin it moves into an intersegmental position. Weiss maintains that 

 the vertical plate and the arch fuse with the vertebra, Bardeen that only 

 the arch does so. 



In this paper I have undertaken to trace the development of the 

 notochord, in the pig, from the time of the appearance of its segmental 

 waves. It was also found necessary to review the formation of the 

 vertebrae in order to determine the exact relation between the noto- 

 chordal waves and other metameric structures. 



The development of the notochord in man, the rabbit, cat, dog and 

 opossum has also been studied, but in less detail. The shape of the 

 intervetebral notochordal expansions has been found to be sufficiently 

 characteristic in each species to justify a brief description of the form 

 of the notochordal enlargements. 



The study has been made possible by the extensive series of marmna- 

 lian embryos in the Harvard Embryological Collection, and the series 

 referred to by number belong to this collection. 



I am greatly indebted to Prof. Charles S. Minot for many helpful 

 suggestions for the prosecution of the work. 



The Formation of the Notochordal Sheaths and of the Pre- 

 cartilaginous vertebrae of the pig. 



The notochord of a very A'oung pig embryo (5.5 mm., H. E. C. !N"os. 

 915, 916, 917) is a dorso-ventrally fiattened rod with major and minor 

 axes approximately 25 and 50 micra long. Each cross section contains 

 about eight wedge-shaped cells whose exposed walls form a thin noto- 

 chordal sheath. As was shown by Dr. Minot in 1907, the notochord 

 and the fioor of the spinal cord of young mammalian embryos are 

 thro^ATi into a series of segmental undulations. In the pig the crests 

 or dorsal curves of the notochord are nearly intersegmental, for they 

 occur very slightly in front of the transverse plane of the intersegmental 

 vessels, the troughs being nearly mid-segmental. 



