360 HISTOLOGY OF THE MORBID PROCESSES. 



quently associated with other neoplasmic tissues of higher differen- 

 tiation, forming mixed tumors; but in such cases the tissues of 

 higher type are not the result of a progressive development on 

 the part of the sarcomatous tissue, for the essential feature of the 

 latter is that it remains in a primitive condition, the formative 

 powers of its cells being chiefly confined to a reproduction of 

 fresh cells, and not to the elaboration of intercellular substances 

 which would convert the tissue into some variety of adult 

 connective tissue. In this respect, as well as in the absence of 

 any natural limitation 'of growth, the sarcomata differ from the 

 tissues of somewhat similar structure which result from the rejuv- 

 enescence of connective tissue in the productive stages of inflam- 

 mation leading to repair. Some forms of sarcoma closely resemble 

 granulation-tissue, for both have the same origin from the cells of 

 the connective tissues ; but the two must be sharply distinguished 

 from each other, for their tendencies and usefulness are extremely 

 different. The formation of granulation-tissue has a definite cause, 

 and it undergoes a progressive differentiation into a dense fibrous 

 tissue, which terminates the process (with the possible, but notable, 

 exception of the development of keloid ; which is, however, not 

 sarcoma). Sarcoma, on the other hand, arises without a well- 

 defined cause, shows no tendency to higher differentiation, and 

 continues to grow without any assignable limitations. A further 

 difference that may aid in the decision of whether an undifferen- 

 tiated tissue of connective-tissue type is sarcoma or due to inflam- 

 matory processes lies in the fact that sarcoma has a tendency to 

 infiltrate the surrounding tissues, while the young connective tissue 

 that results from an inflammatory rejuvenescence has not. 



Sarcomata need not necessarily have the structure of the least 

 differentiated forms of connective tissue. Their cells may show 

 a greater differentiation than is found in that tissue, and there 

 may be a certain amount of intercellular substance of a fibrous 

 or other nature separating the cells. The presence of such a 

 fibrous intercellular substance is an evidence that the forma- 

 tive activity of the cells is not wholly concentrated in the produc- 

 tion of new cells, but is partly diverted to the formation of inter- 

 cellular material. It is therefore a sign of less active growth than 

 would be the case were there no such diversity of activity. The 

 intercellular substances also tend to confine the cells to the growth 

 itself, impeding their penetration into the interstices of the sur- 



