4:50 LECTURE XVIII. 



derived by direct descent from pre-existing epithelial 

 cells. 



So nearly, even at that time, had correct views upon . 

 the subject been obtained, but the blastema theory 

 enthralled men's minds, and we all stood under its in- 

 fluence. Besides, it appeared impossible to point out 

 everywhere in the interior of the tissues the requisite 

 antecedent structures. Not until cellular elements had 

 been shown to exist in connective tissue did it become 

 possible to produce a germinal tissue (Keimgewebe) 

 which is at present everywhere, and from which in the 

 most various organs similar growths may be developed. 

 Now that we know that connective tissue or tis'sues 

 equivalent to it exists in the brain, the liver, the kid- 

 neys, in muscle, cartilage, skin, etc., now there is of 

 course no longer any difficulty in conceiving how the 

 same pathological product may arise in all these appa- 

 rently so dissimiliar structures. No specific blastema of 

 any sort, deposited in all these parts, is at all required, 

 but only the application of a similar stimulus to the con- 

 nective tissue of the different localities. 



Now with regard to the details of this doctrine, allow 

 me in the first place to bring to your notice a concrete 

 example of normal development, which will perhaps be 

 the best calculated to supply you with a picture of the 

 often so complicated processes with which we are here 

 concerned. I choose as my example that organ, in 

 which the process of development is in itself best 

 known, and which at the same time on account of the 

 peculiarity of its structure least admits of misinterpreta- 

 tion, namely the bones. They are too hard and thick 

 for any one to talk about the presence of blastema or 

 exudation in their proper parenchyma. The growth of 

 the bones at the same time affords us direct standards 

 wherewith to compare the different new-formations, 



