EMBRYOLOGY OF THE DUCTLESS GLANDULAR SYSTEM 21 



The middle lobe of the thyroid gland develops from the ventral wall of the 

 entodermal head-gut, the lateral lobes apparently in part from the 

 ventral wall of the fourth pharyngeal pouch. The anterior lobe of 

 the hypophysis and the thyroid gland are phylogenetically older struc- 

 tures and were formerly glands with external secretion that poured 

 their secretions into the intestines. 

 The parathyroid glands are of entodermal origin and develop from the 



dorsal wall of the third and fourth pharyngeal pouches. 

 The thymus gland is also of entodermal origin and develops from the 



ventral wall of the third pharyngeal pouch. 



The pancreatic insular apparatus develops from an extrusion from the 

 pancreatic excretory ducts and according to W ' eichselbaum is of 

 entodermal origin. 

 The suprarenal cortex comes from the epithelium of the body cavity 



and is of mesodermic origin. 



The sexual glands develop from a neighdoring site of the celomic epithe- 

 lium and are therefore also of mesodermal origin; this is generally 

 assumed to be true of the interstitial cells, while the glands of genera- 

 tion, according to the views of many authors, develop directly from 

 the primordial cells. The cells of the suprarenal cortex and the cells 

 of the interstitial glands show a certain morphological similarity. 

 The epiphysis develops from an extrusion of the roof of the third ventricle ; 

 it is therefore in part of neuroectodermal origin; in a clinical respect 

 it shows a certain relationship with the interstitial glands and the 

 suprarenal cortex, so that we can believe that it is in part of meso- 

 dermic origin. 



Hence we may group the ductless glands, according to their origin from 

 the germinal layers, into those of neuroectodermal origin (chromaffin tissue, 

 posterior lobe of the hypophysis), those of ectodermal origin (anterior lobe 

 of the hypophysis), endodermal origin (a) from the head-gut (thyroid), (b) 

 from the branchial derivatives (parathyroid glands, thymus gland), and 

 finally those of mesodermal origin (suprarenal cortex, interstitial glands, 

 pancreatic insular apparatus). The glands of generation seem, on account of 

 their development from the original primordial cells, to take a special place. 

 Although this grouping is incomplete, still it seems to me that important 

 analogies may be derived from it. It is striking that the posterior lobe of the 

 hypophysis (and pars intermedia?) and chromaffin tissue, the active prin- 

 ciples of which, adrenalin and pituitrinum infundibulare, show a certain 

 similarity in their action, are both of neuroectodermic origin, and that the 

 anterior lobe of the hypophysis and the thyroid gland, whose relationship 

 in a physiological and pathological sense has been many times referred to, 

 show phylogenetically a certain relationship in that formerly they were both 

 glands with an external secretion. Finally, it is striking that the interstitial 



