Charles-Sedgwick Minot 251 



set of tissues, which are not duplicated by the tissues of any other germ 

 layer. I have already pointed out on another occasion that the import- 

 ance of the germ layers is as absolute and unvarying in the domain of 

 pathology as in normal differentiation; I need not dwell on that aspect 

 of the question now, but will only repeat the declaration of my belief that 

 the entire teaching of the pathologist as well as of the histologist and 

 anatomist should be based on the doctrine of the germ layers and their 

 specific roles in histogenesis. 



Almost any group of tissues would offer a favorable opportunity for the 

 discussion of genetic classification. We may select those which are 

 differentiated from the embryonic mesenchyma and which are commonly 

 grouped in the adult under the names of the connective and supporting 

 tissues. It is almost superfluous, so much is the genetic point of view 

 neglected, to call your attention to the fact that in our current text-books 

 of histology there is often little or nothing which would enable the student 

 to grasp the relations of these tissues to one another or to understand 

 their genetic relationships. It is true that our knowledge in spite of the 

 great advances of recent years is still too incomplete to justify our assert- 

 ing that the classification which we can now make is final. Kevertheless 

 we can already perhaps attain approximate finality. A very great step 

 in advance was made when the character of the cellular reticulum was 

 established and it was shown that this tissue is different from ordinary 

 connective tissue. It has two principal characteristics : first, the matrix 

 or intercellular substance is nearly or absolutely fluid, so that leucocytes 

 can wander freely in the intercellular spaces of the reticulum; second, 

 the network of original protoplasmic filaments has become directly con- 

 verted into a network preserving more or less the original form, but con- 

 sisting not of protoplasm, but of a new chemical substance, reticulin. 

 Where cellular reticulum is developed, as for instance in the so-called ade- 

 noid tissue, there may be formed from the cells a minimum amount of 

 connective tissue fibrils and of elastic substance, but if we may judge 

 from our present knowledge the cells, which have produced reticulin, 

 preserve but a very small capacity for the production of other elements 

 of connective tissue. Hence, it seems to me that we may well put cellular 

 reticulum in a class by itself, quite apart from the true connective tissues 

 in which the intercellular substance is not mainly fluid and in which there 

 is an abundant development of fibrillar or elastic substance, or of both, 

 and in which, further, reticulin is nearly if not wholly absent. We shall 

 thus come to place all the connective tissues, properly so-called, in a second 

 genetic group. When we follow in the embryo the history of young con- 

 nective tissue, we learn that it undergoes two principal kinds of modifica- 



