290 ALBERT KUNTZ 



As development advances these cellular tracts assume greater 

 prominence and gradually become more fibrous. The interval 

 along the spinal nerve separating the origin of the primarj'- com- 

 municating ramus from the origin of this secondary tract gradu- 

 ally decreases until the two tracts come into close proximity and 

 finally fuse with each other. The secondary tract is not carried 

 forward along the spinal nerve-trunk, but the primary communi- 

 cating ramus is crowded backward somewhat. This is probably 

 due to the formation of the coelom and of the Wolffian bodies. 

 The angle between the primary communicating ramus and the 

 proximal part of the spinal nerve, which in the early stages is an 

 obtuse angle, has now become an acute angle. At the close of the 

 twentieth day of incubation the primary communicating ramus 

 and this secondary tract have come to lie in such close proximity 

 with each other that in some instances they can no longer be dis- 

 tinguished as separate tracts. 



That the origin of the communicating ramus is actually shifted 

 proximally along the spinal nerve-trunk is shown by the curve in 

 the accompanying figure (fig. 5). This curve is based on actual 

 measurements of the interval between the origin of the ventral 

 nerve-root and the origin of the primary communicating ramus 

 in successive stages ranging from the thirteenth to the twenty- 

 eighth day of incubation. The curve is based on the averages of 

 five independent measurements. The figures in the horizontal 

 line indicate the number of days of incubation of the embryos; 

 the figures in the vertical line indicate the relative length of the 

 intervals between the origin of the ventral nerve-roots and the 

 origin of the primary communicating rami in embryos at succes- 

 sive stages of incubation. This curve shows that the interval 

 between the origin of the ventral nerve-root and the origin of the 

 primary communicating ramus increases until the close of the 

 twentieth day of incubation; then decreases until the close of the 

 twenty-second day of incubation, after which it again increases. 

 The somewhat abrupt descent in this curve which occurs between 

 the twentieth and the twenty-third day of incubation, doubtless, 

 indicates an actual proximal displacement of the origin of the pri- 

 mary communicating ramus along the spinal nerve-trunk. 



