356 G. HAROLD DREW 



tinguish between these collaginous strands and the fibrillar pro- 

 cesses of the cytoplasm. When about to divide amitotically, 

 the fibroblasts become shorter and thicker, the cytoplasm van- 

 ishes, and an oval or round nucleus with a reticulated arrangement 

 of the chromatin results ; eventually this splits in two and the two 

 halves separate. The process is shown in fig. 1. 



The details of fibrous tissue formation differ in the early stages 

 according to the degree of irritative action of the foreign body, and 

 the consequent amount of inflammation produced. If the inflam- 

 mation is very slight, as was the case in most of the experiments 

 about to be described, the implanted body is first surrounded by a 

 thin layer of agglutinated blood corpuscles. This is followed by 

 the rapid amitotic division of the fibroblasts in the neighborhood, 

 they lose the typical spindle shape of their nuclei, and the new 

 formed cells consist of rounded or oval nuclei with a scarcely per- 

 ceptible amount of cytoplasm. These rounded cells migrate 

 towards the implanted body, and arrange themselves in layers 

 around it, the nuclei become elongated, and the proportion of 

 cytoplasm increases. Finally a cyst wall of typical fibrous tis- 

 sue is formed, surrounding and completely shutting off the im- 

 planted body. 



METHODS 



Both Pecten maximus and Pecten opercularis can be obtained 

 in large numbers by dredging in the neighborhood of Plymouth. 

 It was found necessary to allow these animals to become accli- 

 matised to living in the laboratory tanks before proceeding to the 

 experimental work. When first placed in the tanks, the mortal- 

 ity was heavy, often amounting to 30 per cent in the first three 

 days, but after the lapse of about a week the survivors appeared 

 to be fully accUmatised to the changed conditions, and often re- 

 mained healthy for some months. 



Experiments on animals whose health was doubtful were of 

 no value, both because the shock consequent on the iniection of 

 the foreign body frequently caused death, and also because the 

 reaction of the tissues was not normal in unhealthy specimens. 

 When a Pecten is healthy, it Ues with the valves of the shell 



