DEVELOPMENT OF THE ADRENALS IN THE TURTLE 87 



These cells do not advance toward the adrenals in continuous 

 cell-columns, nor do they advance along a single path. Appar- 

 ently they advance in small aggregates along various paths 

 through a relatively compact mesenchyme. The processes here 

 involved are probably initiated and controlled by the influence of 

 hormones which are produced by the cells of the cortical sub- 

 stance. The differentiation of 'indifferent' cells into chromaffin 

 cells in the adrenals occurs after they have been associated with 

 the cortical substance for a considerable interval. By this time 

 the cortical cells have probably already assumed a secretory func- 

 tion. It is probable, therefore, that the differentiation of 'in- 

 different' cells into chromaffin cells is also stimulated by the influ- 

 ence of hormones which are produced by the cortical cells. The 

 masses of chromaffin cells which remain apart from the adrenals 

 probably arise from aggregates of 'indifferent' cells whose differ- 

 entiation into neuroblasts or into supporting elements was delayed, 

 but which were located sufficiently near to the adrenals to fall 

 within the sphere of influence of the hormones produced by the 

 cortical cells. 



SUMMARY 



1. The anlagen which give rise to the cortical substance of 

 the adrenals arise in embryos of the Loggerhead turtle (Thalas- 

 sochelys caretta) as buds of cells which proliferate from the peri- 

 toneal epithelium. The chromaffin substance is derived from the 

 anlagen of the prevertebral sympathetic plexuses. These findings 

 agree with the findings of Poll in embryos of Emys europaea and 

 with those of some of the earlier investigators in other types of 

 vertebrates. 



2. The cells which become differentiated into the chromaffin 

 cells, like the majority of the cells which advance peripherally 

 from the cerebro-spinal nervous system into the anlagen of the 

 sympathetic nervous system, are cells of an indifferent type; 

 viz., the 'indifferent' cells of Schaper. 



3. The processes involved in the displacement of the 'indiffer- 

 ent' cells from the anlagen of the prevertebral sympathetic plex- 

 uses toward the adrenals and the differentiation of these cells into 



