THE THYMUS. 



Ill 



menschlicher Embryonen, iii., p. 101, where reference to the literature of these accessory 

 thyroids may be found). 



The bilateral diverbicula, which assist in the formation of the thyroid body, spring 

 from the fourth visceral cleft (Born) (fig. 13f>, //'). They have at first the appearance 

 of simple saccular glands partially encircling the developing larynx (fig. 134; thr'). 

 In front of this they come into connection with the median rudiment, and eventually 

 blend with it. Like that rudiment, they become entirely separated from the hypo- 

 blastic surface from which they have taken origin, their cavity disappears, and they 



fc 



OS. 



Fig. 134. SKETCHES SHOWING THE CONDITION OF THE THYROID AND THYMUS GLANDS IN A HUMAN 



EMBRYO OF ABOUT FIVE WEEKS. (His.) 



A, profile sketch from the left side. 



B, frontal sketch from behind. 



t, tongue ; d, ductus thyreoglossus ; ep, epiglottis ; opposite I, larynx ; tr, trachea ; ce, oesophagus ; 

 thr, median rudiment of thyroid ; thr', lateral rudiment of thyroid ; thm, developing thymus, seen on 

 the left side of B to be connected with a visceral cleft ; ao (in B), ascending aorta ; ao', descending aorta ; 

 c, carotid. 



are converted into ramifying and anastomosing cell-cylinders, between which vascular 

 connective tissue becomes developed. The cell-cylinders subsequently become hol- 

 lowed out, and finally are subdivided by growth of the connective tissue into small 

 vesicles, which gradually become larger from accumulation of colloid in their interior. 



In most Vertebrates, the lateral and median parts of the thyroid remain distinct ; the 

 former are the organs known as supra-pericarcLlal bodies in elasmobranchs, and as accessory 

 thyroids in other animals. Only in mammals do they become united into one organ as 

 in man. 



The thymus is also developed as a growth of the epithelium (hypoblast) of some 

 of the visceral clefts ; in birds from the third and fourth (fig. 135, thym), in reptiles 

 from the second, third, and fourth, and in lower Vertebrates from several clefts (de 



Fig. 135. DIAGRAM SHOWING THE VISCERAL CLEFTS FROM WHICH THE 



THYMUS AND LATERAL PARTS OF THE THYROID ARE DEVELOPED IN 



THE CHICK, (de Meuron. ) 



1, 2, 3, 4, indicate the corresponding visceral clefts ; thym, rudiments of 

 thymus ; thr, median rudiment of thyroid ; thr', lateral rudiments of 

 thyroid. 



Meuron). In mammals the thymus appears as a (bilateral) 



tubular prolongation backwards of the extremity of the 



third visceral cleft (Kolliker), receiving, according to de 



Meuron, an accession from the hypoblast of the fourth cleft, 



as in birds. The tube, which has a narrow lumen, and 



comparatively thick epithelial lining, is surrounded by vascular connective tissue, 



within which numerous lymphoid cells become accumulated, and grows dov/nwards 



along the side and in front of the trachea, where, in mammals, it generally unites 



