128 PROTOZOA AND DISEASE 



(Fig. 51), and finally the tissue at the acme of the process i.e., just 

 at the time at which the central part of the tumour begins to show 

 retrogressive change (Fig. 52). In this way only can a just 

 impression of the whole life of the tumour be traced, in the same way 

 as we trace the inner life of an organism by studying its structure 

 at different periods of its existence, and as the objective features of 

 the human sarcomas have been given in the preceding chapters of 

 this volume. 



The general features of a relatively large lesion are shown in 

 Fig. 49. Camera drawings from parts of this section contained 



FIG. 49. INFECTIVE TUMOUR OF DOGS. (Natural size.) 



The appearance of a complete section through a relatively large lesion. The 

 circle I, at the marginal part, marks the area from which Fig. 50 was drawn ; 

 and the circle 2 shows the part from which Fig. 52 was taken. 



within the circles / and 2 are represented in Figs. 50 and 52 

 respectively. 



In the marginal part (Fig. 50) there is a gradual change from 

 normal fibrous tissue below to the more open cellular structure of 

 the tumour in the upper part. When the same area is carefully 

 examined with a higher power this impression is proved to be 

 correct : the cells of the tumour are found to be connective-tissue 

 cells. They are the pre-existing cells which have changed from the 

 flattened and shrunken resting state to the rounded and succulent 

 granulation-tissue state, and increased in numbers. There is no 

 infiltration of leucocytes. 



Like the alveolar sarcoma of the human breast and the round- 

 celled sarcoma of the human testis described above, these tumours 

 of dogs are granulomata. There are evidences of multiplication 

 of cells at the margin of the tumour ; for although in the area 



