362 B. F. KINGSBURY 



panying growth shiftings, becomes markedly prolonged and 

 regionally differentiated into (1) a head containing the para- 

 thyreoid III and the branchial remnant (Hammar), closely 

 joined to the cervical vesicle; (2) a neck or intermediary portion 

 or cord (thymic cord, Hammar); and (3) a body, the ventral 

 free end or apical portion of the pouch which coming to be 

 located within the thorax undergoes thymic transformation, 

 extending up a varying distance into and beyond the thoracic 

 aperture. The remaining portions of the complex including 

 the cervical vesicle degenerate and disappear, with the exception 

 of the parathyreoid. In the pig, the entire complex — head, 

 neck (intermediary cord, mid cervical segment, cervicothoracic 

 cord) and body (thoracic segment), always excepting the para- 

 thyreoid — undergoes thymic transformation, as Zottermann 

 and Badertscher particularly have shown, although their results 

 are in full accord in this respect with the less extensive studies of 

 earlier investigators. In the guinea-pig, it is equally clear that 

 the entire third complex, excepting the parathyreoid-forming 

 portion, becomes transformed into thymus (H. Rabl '13, Ruben 

 '11). The ventral pocket or prolongation ''remains very weakly 

 developed" (Ruben) or is apparently lacking, according to the 

 description of H. Rabl, and the complex remains purely cervical. 

 In this form, somewhat different conditions of unequal growth, 

 with different grouping of mechanical shiftings, may be in part 

 responsible for the cervical position. Furthermore, the cervical 

 vesicle becomes thymus, as Badertscher and Zottermann have 

 shown in the case of the pig, and Ruben in the guinea-pig. 



It is unnecessary further to multiply illustrations of the varia- 

 bility in extent of development or in position, by considering 

 the results of investigators in the development of the thymus in 

 other mammals (sheep, mole, cat, mouse, etc.) since the well 

 established facts above stated — while they do indicate, as do 

 the numerous observations on lower vertebrates, a marked and 

 extensive tendency to form thymus bodies inherent in the bran- 

 chial region — do not afford any support to the acceptance of 

 a specific and intrinsic anlage for the thymus in the branchial 

 epithelium, which would have to include therefore — to limit the 



