GENERAL CONSIDERATIONS 31 



tion. In every case it is advisable first to study the sequence of events in the 

 struggle of the host against the transplant, which sets in following trans- 

 plantation and ends with the establishment of a new equilibrium of one or 

 another kind. Thereafter, the time of examination should be, as far as possible, 

 a constant factor in all the experiments. The time selected should be such that 

 the effects of the injury, due to the process of transplantation, have disap- 

 peared, but the reactions have not yet progressed so far that finer gradations 

 of the effects in different experiments have become impossible. The latter 

 condition is very important, but it has not received due consideration by some 

 investigators. As a rule, a period of 20 to 30 days following transplantation 

 will be found most suitable for a comparison of the various tissue reactions 

 and for the determination of relationship between the individuality differen- 

 tials of host and graft. If the degree of incompatibility between host and 

 transplant is only slight, a longer time may be required for the lymphocytic 

 accumulations and infiltrations around the graft to become manifest, and in 

 some cases collections of lymphocytes may appear even at a very long time 

 following transplantation. However, in certain transplanted tissues the lym- 

 phocytic infiltration does not increase with increasing length of time after 

 transplantation and there are some indications that in some instances it may 

 even decrease in strength with advancing time. Whether this decrease in the 

 effectiveness of the transplant with increasing time, which is especially 

 noticeable after transplantation of cartilage, is due to a diminution in the 

 amount of homoiotoxins produced or given off in the strange host, or whether 

 it is due to an adaptation of the host to the action of the homoiotoxins, needs 

 further study. 



The choice of tissues to be used varies somewhat in different species. In 

 guinea pig and rat, the simultaneous transplantation of thyroid with adhering 

 parathyroid, of xiphoid cartilage together with the surrounding fat tissue, 

 striated muscle, and bone and bone marrow, will make possible a satisfactory 

 characterization of the relations of the individuality differentials of host and 

 transplant. It may be of advantage to add a separate piece of striated muscle, 

 thymus or salivary gland to the former tissues, all pieces to be implanted at 

 the same time. 



In the case of the mouse, the thyroid is not quite so useful a test tissue as in 

 guinea pig and rat, because in the former species the reaction of the host 

 against the transplant may in some instances consist merely in a shrinking of 

 •the graft, unaccompanied by the lymphocytic reaction which is so fine a 

 reagent in the case of guinea pig and rat. But, also in the mouse a 

 lymphocytic reaction may develop around grafts if incomplete compatibility 

 exists and if the thyroid transplant remains, on the whole, well preserved. 

 However, a shrinking of the thyroid transplant may take place also under other 

 conditions, as when, for instance, the small thyroid of a very young mouse has 

 been used for grafting, or when a part of this organ was injured during the 

 process of transplantation. This multiplicity of factors, bringing about similar 

 results, may make the analysis of the relation of individuality differentials in 

 the mouse more difficult in some experiments. Therefore, in this species it is 



