66 ON THE SUPRA-RENAL, THYMUS, AND THYROID BODIES. 



IV.— ON THE SUPEA-EENAL, THYMUS, AND 

 THYEOID BODIES.— Plate III. 



While engaged, two years ago, in observing the structure 

 of the lymphatic glands, my attention was directed to the 

 thymus, thyroid, and supra-renal bodies ; and I was led to 

 frame a hypothesis, which, althougli afterwards requiring 

 some modification, has, I conceive, nevertheless enabled me to 

 detect, if not the real physiological, at least the morphological 

 signification of these apparently anomalous organs. 



My hypothesis was, that the thyroid, thymus, and supra- 

 renal bodies are the remains of the blastoderma ; the thyroid 

 being a portion of the original cellular substance of the ger- 

 minal membrane grouped around the two principal branches 

 of the omphalo-mesenteric vein ; the supra-renal capsules, 

 constituting other portions grouped around the omphalo- 

 mesenteric arteries ; and the thymus, the intermediate portion 

 of the same membrane arranged along the sides of the em- 

 bryonic visceral cavity. 



Subsequent observations have satisfied me that this hypo- 

 thesis is essentially correct, with the exception of that part of 

 it relating to the thyroid, which body I have now ascertained 

 to be a portion of the membrana intermedia of Eeichert, 

 which remains in connection with anastomosing vessels 

 between the first and second aortic arches, or carotid and sub- 

 clavian arteries. 



In the embryo of the sheep, while the branchial clefts are 

 still open, and for some time afterwards, there is a quantity of 



