THE THYMUS 



171 



According to Hammar (Arch. f. mikr. Anat., Bd. 61, 1903), the lateral pharyngeal recess 

 (of Rosenmueller) is not a persistent portion of the second pouch as His asserted. Lymphocytes 

 appear in the lymphoid tissue of the 

 tonsils in embryos of 140 mm. They 

 take their origin in the mesoderm 

 (Hammar, Maximow). 



Foramen caecum 



THE THYMUS 

 The third, fourth and fifth 

 pharyngeal pouches give rise to 

 a series of ductless glands, of 

 which the thymus is the most 

 important. The thymus anlage 

 appears in 10 mm. embryos as a 

 ventral and medial prolongation 

 of the third pair of pouches (Figs. 

 162 and 163). The ducts con- 

 necting the diverticula with the 

 pharynx soon disappear so that 

 the thymus anlages are set free. 



Palatine tonsil 



Epithelial bodies 



TTjymits 

 anlages 



Post -branchial 



FIG. 163. Diagram in ventral view of the pharynx 

 and pharyngeal pouches, showing the origin of the thymus 

 and thyreoid glands and of the epithelial bodies (modified 

 after Groschuff and Kohn). I, II, III, IV, and V, first, 

 second, third, fourth and fifth pharyngeal pouches. 



At first hollow tubes, they soon lose their cavities, 



their lower ends enlarge and migrate caudally into the thorax passing usually ven- 



FIG. 164. Two reconstructions of the thymus and thyreoid glands. A, in a human embryo of 26 

 mm.; B, in one of 24 mm. (after Tourneux and Verdun). In A the thymus lies in front, in B, behind 

 the left innominate vein, thyr., thyreoid; thym., thymus; par. IV., parathyreoid of fourth pouch; 

 par. III., parathyreoid of third pouch; pyr., pyramidal lobe of thyreoid; thyg., thyreoglossal duct; c.a.; 

 carotid artery; j.v., jugular vein. 



tral to the left vena anonyma. Their upper ends become attentuate and atrophy 

 or may persist as an accessory thymus lobe (Kohn) . The enlarged lower ends of 

 the anlages form the body of the gland, which is thus a paired structure (Fig. 164). 



