86 ALBERT KUNTZ 



stance are, during the earlier stages of development, identical^ 

 as far as may be determined by microscopic observation, with the 

 cells which remain in the sympathetic plexuses. We are forced 

 to the conclusion, therefore, that they are cells of the 'indifferent' 

 type. 



The question now arises ; why do these cells which become asso- 

 ciated with the cortical cells of the adrenals, as well as certain 

 other aggregates which remain apart from the cortical substance, 

 become differentiated into chromaffin cells while other cells en- 

 dowed with the same initial capacity do not? This question we 

 can not answer conclusively at present. It is suggestive, however, 

 that the differentiation of these cells into chromaffin cells takes 

 place comparatively late in the course of development. 



The cells destined to become transformed into chromaffin cells 

 show very little evidence of differentiation before the thirty-sixth 

 day of incubation. Even at the forty-three day stage which was 

 the latest embryonic stage of Thalassochelys caretta at my dis- 

 posal and which falls within a week of the time of hatching, many 

 of the cells of sympathetic origin associated with the cortical 

 substance of the adrenals still remain apparently in their undif- 

 ferentiated condition. Some of the cells of sympathetic origin 

 in the adrenals, however, have assumed a polyhedral form which 

 is the typical form of the chromaffin cells. The cytoplasm of 

 these cells also stains somewhat more intensely than in the earlier 

 stages, but does not as yet present the coarsely granular appear- 

 ance, when stained by the iron-haematoxylin method, which is 

 characteristic of mature chromaffin cells. 



In several earlier papers ('11 a, '11 b, '11 d), evidence was pre- 

 sented in support of the theory that the processes involved in the 

 peripheral displacement of the cells giving rise to the anlagen of 

 the sympathetic nervous system are stimulated and controlled 

 by the influence of hormones which are produced in the regions 

 toward which the cells advance. The displacement of the cells 

 of sympathetic origin which give rise to the chromaffin cells from 

 the sympathetic plexuses into the adrenals must, doubtless, be 

 accounted for in a similar manner. This displacement can not 

 be accounted for by the mechanical processes involved in growth. 



