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IOWA ACADEMY OF SCIENCE 
adrenal bud within the thin mesodermic capsule. These cells are cells of an 
indifferent character, viz., the “indifferent” cells of Schaper and are destined 
to become the chromaffin substance. 
Older embryos, 25-35 mm. in length, show inward migration of these in- 
different cells. Cords and masses of them push in between the aggregates of 
cortical cells into which the bud is now broken. The cortical cells are char- 
acterized by larger size and by a difference in their staining properties. These 
migrating cells continue to displace the cortical matter until in embryos 80 mm. 
in length indifferent or chromaffin masses predominate in the central region, 
whil« in the embryo 180 mm. in length little but chromaflGln material is left 
in the medullary portion. 
While surrounding the bud and early in the migration, the cells from the 
sympathetic masses are similar in size and stain to those of the adjacent 
sympathetic aggregates. The beginning of differentiation of these indifferent 
sympathetic cells into chromaffin cells is coincident with migration. 
After the embryos have attained about 40 mm. in length, the capsule closes 
around the gland except along the mesial surface. This portion thickens and 
the capsule is entire in embryos 90 mm. in length which suggests that further 
entrance of migratory cells is prohibited. This would leave the above described 
entrance probably the only one for such cells and since only cells of sympathetic 
origin enter in this manner we are forced to the conclusion that the chromaffin 
substance of the adrenals is derived from the sympathetic system. 
