350 G. HAROLD DREW 



pith and other foreign bodies which are presumably unaffected 

 by the body fluids of Pecten. In these experiments it was noticed 

 that the reaction of the tissues to the implantation of portions of 

 the ovary presented, in the later stages, marked differences from 

 the reaction to any of the other tissues or foreign bodies employed, 

 and it was with the object of explaining these differences that the 

 work here described was undertaken. 



Briefly summarised, my results show that after implantation 

 of a portion of the ripe ovary into the adductor muscle, a layer 

 of fibroblasts is formed around it, and coincidently the ovarian 

 tissue is invaded by phagocytes and degenerates. After the lapse 

 of about six days no trace of organised ovarian tissue remains, 

 but there is left a cyst surrounded by fibroblasts, and containing 

 blood corpuscles and a quantity of small granules having the 

 orange color of the yolk substance. After the lapse of about 20 

 days more, the innermost fibroblasts gradually change their 

 shape, and form a layer resembling columnar epithelial cells, which 

 later become ciliated. Eventually the whole cyst becomes lined 

 with well defined ciliated epithelium, which persists at least for 

 120 days, which is the longest period I have yet succeeded in keep- 

 ing the animals alive in the experimental tanks. I consider that 

 there is some evidence to show that this change of the fibroblasts 

 into ciliated epithelium is a reaction to the presence of some defi- 

 nite chemical substance within the cyst. 



In the vertebrate many cases of the change of a tissue of one 

 type into another type are known to occur, and to such changes 

 the name Metaplasia has been given. According to Adami, ('08) 

 they have this in common, that ''Epithelial (epiblastic andhypo- 

 blastic) tissues can only be converted into other forms of epi- 

 thelial tissue, and one form of mesoblastic into another form of 

 mesoblastic." 



Such examples as the conversion of cartilage into bone, or of 

 connective tissue cells into fat, are cases of physiological meta- 

 plasia of mesoblastic tissues, and it is to be noted that these changes 

 occur in the adult and not only in the course of development. In- 

 stances of pathological metaplasia are numerous. Thus among 

 the epithelial tissues we have the change of the columnar ciliated 



