DEVELOPMENT OF THE ADRENALS IN THE TURTLE 81 



farther removed from the aorta and numerous small cell-groups 

 may be traced from the larger cell-aggregates constituting the 

 anlagen of the prevertebral sympathetic plexuses toward the ad- 

 renal anlagen (fig. 6, cc). Many of these cell-groups may be 

 observed closely approximated to the dorsal and the mesial surf aces 

 of the adrenal anlagen, while not infrequently such cells may be 

 observed just beneath the surface of the adrenal anlage among the 

 cortical cells (fig. 6, cc'). 



There is no observable difference at this stage between the cells 

 of sympathetic, origin which advance toward the anlagen of the 

 adrenals and the cells which remain within the sympathetic 

 plexuses. That these are the cells, however, which give rise to 

 the chromaffin substance associated with the cortical substance 

 in the adrenal organs can not be doubted. These cells not only 

 become aggregated at the dorsal and the mesial surfaces of the 

 adrenal anlagen, but, as development advances, groups of these 

 cells may be found completely encircling the cortical substance of 

 the adrenals and penetrating into the spaces which appear between 

 the aggregates of the cortical cells as the latter approach more and 

 more closely the arrangement which is typical of the adrenal 

 glands in the mature state. Furthermore, sympathetic cells 

 do not advance farther ventrally into the mesentery at this level, 

 and, as the writer has shown in the earlier paper referred to above, 

 the cells which give rise to the sympathetic plexuses in the walls 

 of the digestive tube are derived from other sources; viz., the 

 hind-brain and the vagus ganglia. 



Later development 



The record of the later development of the adrenals is essentially 

 a record of the further growth of the glands, the rearrangement 

 of the cells, the adjustment of the chromaffin substance to the 

 cortical substance and the differentiation of the elements derived 

 from the sympathetic nervous system into typical chromaffin 

 cells. 



During the later stages of incubation, the development of the 

 adrenals advances comparatively slowly. The adrenal zone be- 



