ROLE OF THE THYMUS ^ 385 



use of thymectomized newborn mice (Miller, 1962). It then 

 became evident that the thymus at an early stage in life plays a 

 very important part in the development of immunological 

 response. A prehminary commuiiication of this work has been 

 published (Miller, 1961). 



Materials and methods 



Mice 



Mice of the following inbred strains were used: Ak (H-2^), 

 C3H (H-2^), C57BL (H-2'^), BALB/c (H-2^), DBA/2 (hV), 

 T6 (?H-2^), and (Akx T6) F^ hybrids. 



Thymectottiy 



The mice were thymectomized within a few hours of birth 

 unless otherwise indicated. The operation was performed under 

 light ether anaesthesia. Part of the sternum was excised and the 

 thymic lobes were sucked out of the thorax by means of a glass 

 pipette connected to a negative pressure system. Three or four 

 interrupted black silk sutures were used to close the skin edges. 

 The immediate operative mortality was no higher than 15 per 

 cent but the mortality from cannibalism or neglect was con- 

 siderable. Control littermates were sham-thymectomized, that is 

 they underwent the full operative procedure including excision 

 of part of the sternum but the thymic lobes were left intact. 



Thymus grafting 



Thymuses from newborn donor mice were removed aseptically 

 and introduced by a sterile trocar into the subcutaneous tissues 

 under the right axilla of two- to three-week-old recipients. 



Skin grafting 



Skin grafts from i- to 2-month-old mice of either sex or from 

 2- to 3 -week-old Wistar rats of either sex were transplanted to 



