148 D. W. H. Barnes and J. F. Loutit 



invoked in view of the second line of approach to be reported. 

 This is the use of marked chromosomes in the donated 

 material. Two different markers have been employed (Ford 

 et al, 1956). 



(I) The rat chromosomes. It has already been noted that 

 administration of heterologous material can allow the lethally 

 irradiated mouse to survive (Lorenz et al., 1952; Congdon and 

 Lorenz, 1954). We have lately been able to repeat this result 

 using as donors inbred rats from our colony which stems from 

 the Wistar strain. Our CBA mice are given 950 r as before 

 and then injected intravenously with a suspension of bone 

 marrow obtained from the two femurs of a young rat. The 

 recipient CBA mouse has cells with a complement of 40 

 chromosomes with terminal centromeres (Fig. 1). The rat has 

 cells which contain 42 chromosomes, a number of which have 

 a characteristic cruciform appearance in metaphase (Fig. 2), 

 since their centromeres occupy a central position. In a squash 

 preparation it is therefore easy to differentiate those cells in 

 metaphase which are derived from the host, i.e. the mouse, 

 from those which stem from the donor-rat. 



(II) Mouse-translocation T6. Carter, Lyon and Phillips 

 (1955), by irradiating the testes of male mice and promptly 

 breeding from them, were able to select offspring that were 

 semi-sterile as a result of inheriting a pair of translocated 

 chromosomes from the irradiated parent. Ford and Hamerton 

 in this laboratory examined cytologically the available stocks 

 carrying these translocations and showed that one (denoted 

 by Carter and co-workers as T6) had a chromosome which was 

 readily differentiated at metaphase from any of the normal 

 chromosomes of the mouse. It was about half the length of 

 the smallest normal chromosome (Fig. 3). Dr. Lyon has bred 

 for us young mice carrying this translocation in the hetero- 

 zygous state and we have used the spleens of these mice aged 

 7-10 days as donor material for the irradiated CBA mice. 



The irradiated CBA mice, treated with either the rat 

 bone marrow or the spleen of the T6 mouse, have been sacri- 

 ficed at intervals after irradiation and treatment and their 



