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



where more than one animal has been sacrificed, the cell- 

 counts reported in Table III are the sum of the individual 

 counts. On the one occasion when one differed from its mate 

 or mates the count on that animal is recorded separately. 

 Preparations were made of liver but no mitoses were seen. 

 Testes were examined but were completely atrophic as judged 

 by the naked eye and without mitotic figures in the cytological 

 preparations. 



The exceptions to the broad generalization made above are 

 as follows: 



(i) In the process of making the squash some cells are 

 violently disrupted and the full complement of 40 chromo- 

 somes of the mouse's cell or 42 of the rat's cell may not be 

 traced. While the rat's cell may be identified by the charac- 

 teristic cruciform members of the set, even in the absence 

 of a full count of chromosomes, to diagnose with certainty 

 in the homologous transfer the presence or absence of the 

 marker, necessitates the visualization of all 40 chromosomes. 

 In some cells with counts of less than 40 chromosomes 

 the marker was not seen. These cells have been scored as 

 "doubtful". 



(ii) Cells may not be sufficiently well spread for all 40 

 chromosomes to be separated and seen. If the marker is not 

 identified, it is not possible to say whether such a cell is from 

 donor or host and the cell is also recorded as "doubtful". 



(iii) Occasional cells are seen in the early days after irradia- 

 tion with multiple lesions of the chromosomes of the kind 

 well known to be induced by radiation e.g. dicentric chromo- 

 somes and acentric fragments. These cells can be attributed 

 to the host, but they do not persist after the first few days 

 (Fig. 4). . 



(iv) While in the early days and weeks after such irradia- 

 tion it appears as if the great majority at least of the cells in 

 division are attributable to the donor, it may well be that in 

 the course of time the host's tissues under investigation will 

 show some recovery. Thus, after seven weeks in the case of 

 the heterologous transfer of rat bone marrow to mouse, we 



