272 Cc. M. JACKSON 
of the suprarenal in animals after the earlier postnatal stages 
usually occur only in the immediate neighborhood of the cortico- 
medullary border. Those originally in the central portion of the 
medulla have usually undergone complete absorption. Flint 
(00), Pellegrino (04), and others have described cortical islands 
in the medulla of the adult suprarenal, and they occur also in the 
rat; but these probably represent the results of later cortical ero- 
sion, rather than persistent embryonic remnants. 
The exact nature of this absorptive process is uncertain. It 
does not appear to be a mere pressure atrophy, due to the expan- 
sion of the cortex, although this may be a factor of subordinate 
importance in the process. . It would appear to be rather a chem- 
ical process of obscure nature, possibly a result of the contact 
between the essentially alien cortical and medullary tissues, not 
yet fully adapted to each other in their comparatively recent 
phylogenetic association. The observation by Lewis and Pap- 
penheimer (10) of similar involutional changes in accessory Su- 
prarenals composed of cortical tissue only would seem to invali- 
date this theory, but this point needs further investigation. It 
does not appear probable that the absorption of the cortex at the 
corticomedullary border is of functional significance, as claimed 
by Gottschau (793) and Mulon (02, ’03. a, 03 b, ’05 a, 705 b, 
1D): 
If the inner zone of the cortex is subject to continued erosion 
and absorption during the morphogenesis of the suprarenal, it is 
evident that the zone must be constantly regenerated. As has 
been previously shown, the abundant evidence in various animals 
clearly establishes the fact that although during embryonic and 
fetal periods cell division occurs throughout the cortex, during 
postnatal development it becomes progressively restricted to the 
outer region of the cortex in general, and to the zona glomerulosa 
in particular. There is therefore during the postnatal growth 
period a continued renewal of the suprarenal cortex, the cyto- 
morphosis of the cells including an origin in or near the outer 
zone (glomerulosa), a descent through the middle zone (fascicu- 
lata), and a final atrophy, absorption and removal in the inner 
zone (reticularis) at the corticomedullary border. The process 
