438 J. A. BADERTSCHER 



who have extended their investigations over a comparatively 

 wide range of species and classes of animals have found that the 

 developmental processes involved are practically the same for 

 the different types of animals investigated. 



While the investigation presented in this paper deals with 

 the histogenesis of the thymus as a whole, special consideration 

 is given, (1) to the origin and nature of the small round cells, 

 and (2) to the origin of free erythrocytes and eosinophile cells 

 that are present in both the interlobular septa and the paren- 

 chyma of the thymus in later developmental stages. Though 

 comparatively little attention has been paid to the develop- 

 ment of the reticulum and the thymic bodies, they nevertheless 

 have received a consideration sufficient to determine their origin. 



MATERIAL AND METHODS 



The material used for the histogenesis of the thymus was 

 collected in great abundance at a packing house. Often the 

 embryos still showed signs of life while they were being measured 

 and prepared for the fixing fluid. The upper jaw, the cranium, 

 and the posterior thoracic wall were removed from embryos 

 from 10 to 20 mm. in length. The part containing the thymus 

 was thus made comparativel}^ small and fixed well. Embryos 

 from 20 to 55 mm. in length were treated in a similar manner 

 and in addition the sides were trimmed and the cervical vertebrae 

 removed in order to reduce the size of the piece. From embryos 

 ranging from 60 to 165 mm. in length only portions of the thymus 

 with some of the surrounding tissues were removed. From all 

 these stages the entire superficial thjmius and thymus head, 

 and parts of the mid-cervical and thoracic portions were pro- 

 cured. From embryos 180 to 280 mm. in length (full term) 

 the superficial thymus and portions of the thymus head and mid- 

 cervical segment were removed. The left thymus was usually 

 selected in those stages from which only a portion of the organ 

 was removed. The lengths in millimeters of the different 

 developmental stages of which the thymus was prepared for a 

 study of its histogenesis are as follows: 17, 20, 23, 25, 26, 27, 



