DEVELOPMENT OF THE ADRENALS IN THE TURTLE 79 



are now located below the middle level of the aorta, while in the 

 posterior region they lie close to the ventro-lateral aspects of the 

 latter and are not far removed from the peritoneal epithelium in 

 the angle between the mesentery and the mesonephros. The 

 primary arrangement of the cells in irregular rows is no longer 

 apparent. At this stage, however, the cells seem to be arranged 

 in more or less regular concentric layers. 



At the close of the thirteenth day of incubation, the aorta has 

 become relatively smaller than in the preceding stages and the 

 internal organs are located farther ventrally with respect to the 

 former. In the anterior region the adrenal anlagen now lie along 

 the ventro-lateral aspects of the aorta, while farther posteriorly 

 they lie distinctly below its ventral level (fig. 5, ad) . In the pos- 

 terior region the anlagen of the adrenals have become somewhat 

 compressed dorso-ventrally and appear more or less oval iii trans- 

 verse sections. The mesial surfaces of the right and the left 

 adrenal have now become closely approximated to each other in 

 this region. Denser aggregates of cells may now be observed 

 scattered irregularly throughout the section of the adrenal anlage. 

 This condition, doubtless, represents the initial stage in the dif- 

 ferentiation leading to the characteristic arrangement of the cells 

 in the cortical substance of the adrenal gland. 



After the close of the thirteenth day of incubation, the adrenal 

 anlagen become somewhat farther removed from the aorta. They 

 increase in size materially and the cells assume a more definite 

 arrangement. At the close of the thirteenth day, the denser 

 aggregates of cells noted above have become more conspicuous 

 and are variously connected with each other by strands of closely 

 aggregated cells (fig. 6, ad). The meshes within this complex 

 stain less intensely than the denser areas and contain relatively 

 few cells. 



The observations thus far recorded agree in all essentials with 

 the observations of Poll ('04, '06) on the development of the 

 cortical substance of the adrenals in Emys europaea. 



Chromaffin substance. The above observations pertain only 

 to those parts of the adrenal anlagen which arise from the peri- 

 toneal epithelium and give rise to what is commonly known as 



