TRANSPLANTATION OF TISSUES 1143 



been studied. Normally, a migration of tissue elements is only 

 witnessed in the adult in the case of cells moving with the circulating 

 liquids, or endowed with the power of amoeboid movement. Under 

 pathological conditions fragments of tissue, such as tumour cells, 

 may be carried by the blood or lymph to distant parts, and, settling 

 there, may undergo development (forming metastases). In the 

 embryo the slow migration of tissue elements is a process which 

 is responsible for some of the anatomical pecularities of the adult. 

 The migration of the ovum from the ovary is the starting-point of 

 the process of reproduction. The artificial displacement of tissues 

 within the body of one and the same animal (auto-transplantation 

 or autografting) can be successfully accomplished in the case of 

 most normal organs and tissues, and also in the case of most tumours. 

 Instances have already been given in speaking of the endocrine 

 functions of the ovary, thyroid, spleen, thymus, etc. (Chap. XL). 

 A small piece of tissue or sometimes a small organ is simply inserted 

 in its new situation without provision for the immediate establish- 

 ment of a circulation. Necrosis of the central portion occurs, but 

 the peripheral zone soon becomes vascular, the graft ' takes ' and 

 under suitable physiological conditions grows. Hetero-transplan- 

 tation, or grafting between animals of different species, does not 

 succeed. Homceo-transplantation, or grafting from one animal to 

 another of the same species, is in the case of normal tissues success- 

 ful only in rather rare instances. In most cases, although some 

 of the homceo-grafted tissue may remain alive for some time, or 

 even begin to grow, its growth is soon checked, and it is eventually 

 absorbed. Certain tumours, however, can be readily grafted from 

 one animal to another of the same species, and can live and grow in 

 the new environment. 



The difference in the fate of auto- and homoeografts illustrates in a 

 striking way the important chemical and metabolic differences which 

 exist, not only between different species, but between individuals of the 

 same species. In general, a piece of tissue from a rabbit is treated as 

 an ' unclean ' thing, which must eventually be cast out, not only when 

 it is introduced into the body of a dog, but when it is introduced into the 

 body of another rabbit. It is unable to adapt itself to its new en- 

 vironment, and soon perishes. But a bit of thyroid, of adrenal cortex, 

 or of uterus, may easily settle down as a successful colonist in very out- 

 landish places within the body of the animal to which it belongs. An 

 invasion of lymphocytes, and an ingrowth of fibroblasts, causing develop- 

 ment of bands of connective tissue, have been regarded by some observers 

 as the immediate causes of the failure of homoeografts to grow. It is 

 fully as probable, however, that the lymphocytes gather around and 

 invade the graft, and that the connective tissue trabeculse appear in it, 

 because it has already been injured by the antibodies of the host, or, if 

 not by specific antibodies, then simply by exposure to the more or less 

 altered metabolic conditions, which it is unable to face successfully. 

 The difference between the autograft and the homoeograft has not yet 

 been sufficiently taken account of by surgeons, e.g., in connection with 



