Allen, Association in the Guinea Pig. 347 



as are to be found near the margins of the ventral columns. 



3. At all levels there are many medullated fibers passing 

 in every direction throughout the gray substance. 



//. Develop7nent of the Medulla Spinalis frojn Birth to Maturity. 



Cervical Level- 

 Between birth and the third or fourth days the formation of 

 medullary substance in the medulla spinalis is apparently at a 

 standstill. After the third day the light areas which have just 

 been described, show a rapid darkening, so that before the 

 eleventh day the meduUation of the whole section has become 

 practically uniform, the light areas being closely packed with 

 medullated fibers. These fibers appear to have a smaller aver- 

 age diameter than the fibers of the neighboring funiculi, but 

 their number is sufficient to render these areas as dark as those 

 about them. Between eleven and thirty days the only notice- 

 able change, besides increase in the area of the whole transverse 

 section, is the still further darkening of the areas a, b and c. 



In order to compare the white rat of thirty-five days with 

 a guinea pig at approximately the same stage of development 

 (thirty days), the transverse sections from the three levels of a 

 thirty-day guinea pig are reproduced (figs. 5, 6, 7, plate V). 



Upon reference to the figure of the cervical level it will 

 be seen that the whole field has become darker than at birth by 

 the great increase in number and in size of medullated fibers. 

 The gray substance is traversed by a relatively larger number 

 of fibers than is present at birth. The drawing of the thirty-day 

 cervical section shows particularly well the cork-screw arrange- 

 ment of the intra-meduUary portion of the fibers passing into 

 the gray substance from the zone of entering roots. 



A small light area is to be found in the thirty-day cord 

 also, at the ventro-lateral tip of the substantia gelatinosa. About 

 the tip of the substantia gelatinosa are many fibers running in 

 the plane of the section. 



In the cervical cord of the adult (fig. 8, Plate V) the num- 

 ber of medullated fibers is largely increased, the white substance 

 appearing nearly black. Throughout the entire section 



[55] 



