DEVELOPMENT OF THE THYMUS 331 



From the thymus head the intermediary cord {C.i.) and the 

 mid-cervical segment (S.m.) extend in a meso-ventral direction 

 to the anterior aperture of the thorax ventral to the trachea. 

 In comparatively early stages they are more or less tortuous as 

 represented in figure A, while in later stages their course is 

 nearly straight. The mid-cervical segment in early stages is 

 short and lies immediately anterior to the thorax while the inter- 

 mediary cord is comparatively long as represented in figure A. 

 As development proceeds the mid-cervical segment gradually 

 becomes longer while the intermediary cord becomes shorter as 

 represented in the figures. 



The cervico-thoracic segment (C.ct.) in all stages is short and 

 of a comparatively small diameter. It Ues in the extreme ven- 

 tral portion of the anterior aperture of the thorax. In early 

 stages the cords of the right and left thymus lie closely together 

 and in later stages they fuse with each other. 



The thoracic segment (S.th.) in later developmental stages is 

 composed of the thoracic portions of both the right and left 

 thymus which have fused in this region. In embryos about 

 105 mm. in length, and later stages, it is spread over the antero- 

 ventral surface of the left side of the pericardium. The swinging 

 of the right segment toward the left side is already noticeable 

 in a 42 mm. embryo. This segment is thickest along the median 

 line (3.5 to 4 mm. in full term embryos) and gradually tapers 

 down to a thin irregular edge. 



The connecting band was present on both sides in the majority 

 of specmiens examined. It may, however, be absent either on 

 one or on both sides. Its rupture is apparently due to the growth 

 in length of the thymus not keeping pace with the growth in 

 length of the neck. The expanded caudal portion of the thymus 

 being firmly anchored in the anterior portion of the thoracic cavity, 

 on account of the unequal rate of growth between the neck and 

 the thymus, will exert a pull on that portion of the organ in the 

 neck and thus greatly attenuate or tear the connecting band. 

 Also there is thus a stress exerted on the hypoglossal nerve which 

 apparently tends slightly to change its direction, as stated in 

 the description of the thymus in 21.5 mm. embryo (p. 325). 



