DEVELOPMENT OF HUMAN PHARYNX 367 



In this connection the writer, on his part, desires to offer the 

 comment that the intrusion of a teleology, while in many instances 

 in biological work apparently justifiable as an avoidance of com- 

 plex paraphrasing and explanation, is in such instances as the 

 above quite unnecessary. Before leaving the question of a 

 iymphocytopoietic' reaction of mesenchyme (connective tissue) 

 in the presence of regressive structures I wish to mention some 

 other aspects. First, every histologist is familiar .with the 

 circumscribed small mononuclear cell (lymphocytic) accumula- 

 tions, more or less compact, occasionally encountered in glands 

 of the most diverse character — kidney, (thyreoid) lachrymal 

 gland, salivary glands, as well as in minor glands of the res- 

 piratory and digestive systems — attended bj^ the degeneration 

 and infiltration of a certain number of the epithelial acini. 



In the transplantation of tissues, where in some instances 

 they 'take' and grow, and m other cases (as in most homoi- 

 transplants, Loeb) they fail to do so and undergo regressive 

 change and disappear, peculiar opportunity is given for the 

 examination of the relation of regressive structure to its en- 

 vironment. The statement of Leo Loeb ('15) in summing up 

 the results of many years of investigation in this field are so 

 pertinent that I venture to quote in some detail: 



A certain metabolic activity on the part of parenchyma of various 

 organs determines the attitude of lymphocytes and of fibroblasts toward 

 the parenchyma. If the activity is normal, lymphocytes do not or 

 only to a slight extent enter the parenchyma. The connective tissue 



is held in a definite state of activity (p. 728) While the 



vitality of the tisses has not been essentially impaired after homoio- 

 transplantation, and they may even grow, and the metabolic changes 

 they underwent in the different chemical environment did not there- 

 fore markedly interfere with their power to live or even propagate, 

 these metabolic changes lead to a new condition in the host tissue, 

 which secondarily brings about a destruction of the transplanted piece, 

 namely, these metabolic changes cause (a) an increased activity on 

 the part of small monocuclear cells (probably lymphocytes) and (b) 

 a destructive activity on the part of the connective tissue of the host. 

 The mononuclear cells collect around the transplanted tissue, penetrate 

 into it and destroy its cells. . . . We may conclude therefore that 

 the metabolic changes taking place in tissues after homoiotransplanta- 

 tion stimulate the activity of the lymphocytes and cause the altered 



