132 INTERNAL SECRETION 



on, the buds become detached from the parent membrane and 

 assume the status of interrenal organs which, in the form of 

 isolated cell-agglomerations, lie free in the supporting tissue and, 

 by their change of position, acquire fresh topographical relation- 

 ships to the surrounding organs. At the same time, as growth 

 continues, processes of degeneration and of coalescence take place, 

 leading, in the former case, to a reduction in the length of the 

 system, which originally extended over comparatively large por- 

 tions of the body of the embryo ; and in the latter, to the partial 

 or complete negation of the primary discontinuity of the system. 

 Thus, vigorous growth and the coalescence of separate buds is the 

 primary origin of the principal organs, while accessory organs are 

 primarily the result of imperfect involution. Accessory bodies 

 may, however, be formed secondarily at a later date by fission of 

 the principal organs. 



The embryonal interrenal cells undergo further histological 

 and cytological differentiation until the specific structure is 

 assumed. In cyclostomata and in fish, the interrenal system 

 remains in this condition of independence. 



The adrenal system is. derived from the ectoderm ; it shares its 

 primordial beginning with the sympathetic system and develops 

 as a part of it. In the course of the histogenesis of the sympathetic 

 ganglia, two different cell-types are formed from the apparently 

 homogeneous and undifferentiated seat of origin of the primitive 

 ganglia. Of these, one is the prototype of the sympathetic nerve- 

 cells, while the other includes certain specific elements of a peculiar 

 character which are the first chromaffine cells (phasochromoblasts). 

 The latter occur either singly or in larger or smaller groups in the 

 midst of the nervous tissue and not divided from it. The 

 apparently homogeneous primitive beginning may, however, 

 divide into two in such a manner that the chromaffine tissue lies 

 within or near a sympathetic ganglion, and may occasionally be 

 separated from it by a sheath of connective tissue. In the Selachii, 

 the elements of the adrenal system remain stationary at this stage 

 of development, and are represented by the so-called suprarenal 

 bodies of this order. Finally, there may be a complete topo- 

 . graphical separation of the adrenal elements from the sympathetic 

 system, the chromaffine bodies, as soon as they become separate 

 structures, showing only very slight traces of their genetic asso- 

 ciation with the sympathetic. Separation of all three kinds is 

 seen in amniota and in amphibia. In these orders the adrenal 

 system consists of depositions of chromaffine tissue within the 

 sympathetic ganglia and plexuses ; of deposits of chromaffine 

 bodies attached to the sympathetic ; and of relatively autonomous 

 structures like the so-called accessory organs. The slight dis- 

 position to a separate existence by which the organs of the adrenal 

 system in general are characterized, is also shown by the fact that 

 portions of this tissue associate themselves at an early develop- 



