Allen, Association in the Gtiinea Pig. 351 



2. Before eleven days the first two areas are medullated 

 uniformly with the surrounding regions. The area in the lateral 

 funiculus has received many new fibers and can be distinguished 

 only as a narrow light zone bordering upon the lateral apex of 

 the substantia gelatinosa. 



3. By thirty days the two areas in the dorsal funiculus 

 just distinguished as medullating late have become darker than 

 the surrounding region. In the adult these areas are relatively 

 still darker than at thirty days. 



4. In the adult the light area in the lateral funiculus still 

 has small fibers, many of which pass longitudinally within the 

 limits of the substantia gelatinosa itself 



5. Since the lumbar level at birth does not possess a fas- 

 culus gracilis it lacks one light area mentioned for the levels 

 above. The other two light areas are present, though less 

 well marked than at the higher levels. 



6. The ventral half of the pyramidal area soon becomes 

 well medullated. but its dorsal half remains poorly medullated 

 for a longer time than at the higher levels, since even at thirty 

 •days it is still a little lighter than the adjacent white substance. 



7. On the whole the cord of the guinea pig at birth is 

 very well medullated in all its regions. 



Jncrease in Area of Cross Sections of the Medulla Spinalis from 

 Birth to Maturity. 



The changes in the spinal cord which have been described 

 above are such as are apparent by inspection of the various fun- 

 iculi represented in the drawings. Contemporaneous with the 

 darkening of the section by the increase in the size and the 

 number of medullated fibers there has been an enlargement of 

 the total area of the cord. 



A selection of typical sections was made, and their transverse 

 areas ascertained with the planimeter. The results are indi- 

 cated in the foUowingf table : 



[59] 



