DEVELOPMENT OF THE THYMUS 327 



ficial thymus and extends as a greatly attenuated cord of cells 

 toward the hypoglossal nerve. The general features of the entire 

 thymus at this stage so closely resemble those of the 26 mm. 

 embryo that a detailed description is unnecessary. The only 

 difference of importance is the greater size of the organ as a 

 whole and of the epithelial buds from the main stem. 



Embryo of 40 mm. The connecting band is continuous around 

 the hypoglossal nerve on both sides. The parathyroids are 

 slightly elongated and lie along the dorso-mesial side of the 

 central third of the thymus head. Many of the primary epi- 

 thelial buds of the enlarged segments (the superficial thymus, 

 thymus heads, mid-cervical and thoracic segments) have sent 

 out processes, thus marking the beginning of secondaiy lobula- 

 tion. The intermediary cords are greatly attenuated and show 

 no signs of budding. The transition from the thj^mus head to 

 the intermediary cords and from the latter to the mid-cervical 

 segment is very abrupt. The cervico-thoracic cords are short 

 and lie near each other a short distance ventral to the trachear. 

 The thoracic segments of both the right and left thymus are 

 now fused along the greater part of their median plane. They 

 are a little larger than those in the previous stage and have the 

 same general position over the anterior and ventral portion of 

 the pericardium. 



Embryo of 52 mm. The connecting band on each side loops 

 over the hypoglossal nerve and connects the thymus head with 

 the superficial thymus. The one on the left side is a compara- 

 tively large and irregularly modelled cord of differentiated thymic 

 tissue while the one on the right side is a slender and greatly 

 attenuated cord of epithelial cells. The intermediary cords are 

 still greatly attenuated cords of epithelial cells but are now 

 studded here and there with small epithelial buds. The left 

 cervico-thoracic cord is still slender with a nearly uniform diam- 

 eter while the right one is much larger and has undergone lobu- 

 lation. Both are now of differentiated thymic structure. The 

 enlarged segments of the thymus are appreciably larger than 

 those in the 40 mm. embryo. They have undergone extended 



