326 J. A. BADERTSCHER 



the ^thoracic segment' which lies in the anterior portion of the 

 thorax and is spread over a portion of the pericardium; and 

 (7) the 'cervico-thoracic cord' which unites the mid-cervical 

 segment to the thoracic segment. This system of nomenclature, 

 for which we are indebted to Kastschenko, Zotterman, and Bell, 

 will be used in the discussion of all the later developmental 

 stages. 



The different regions of the thymus described in the 21.5 mm. 

 embryo were examined microscopically in the following five 

 stages. These will be briefly described in order to present a 

 more complete developmental history up to a 95 mm. embryo, 

 which was the oldest stage in which a part of the pharyngeal 

 region was reconstructed. 



Embryo of 26 mm. The thymus as a whole is considerably 

 larger than in the preceding stage. The surface of the super- 

 ficial thymus, the thymus head, and the thoracic segment has 

 become very irregular due to outgrowths of epithelial buds from 

 the main stem. This budding represents the beginning of lobu- 

 lation and had already started in the preceding stage. Lobula- 

 tion of the mid-cervical segment has just begun. The super- 

 ficial thymus extends only shghtly farther caudally from the 

 hypoglossal nerve than in the preceding stage. The connecting 

 band on the right side has disappeared but the superficial thymus 

 has retained its usual topographical relation to the thymus head. 

 From the parathyroid body the general direction of the thymus 

 is in a caudo-mesia land ventral dhection. The intermediary 

 cord on the right side is only a very slender cord of cells while 

 that of the left side has a considerably greater diameter. The 

 cervico-thoracic cords are short and have a uniform diameter 

 of small dimension. The thoracic segments lie in contact with 

 the anterior and ventral portion of the pericardium. The two 

 segments lie close together and have fused in some places along 

 their median sides. 



Embryo of 32 mm. The connecting band on the left side is 

 broken. No traces of it can be seen in connection with the 

 thymus head but a remnant of it is still attached to the super- 



