dob PROVISIONAL HYPOTHESIS Chap. XXVII. 



Mr. Lawson Tait refers to a tumour in which "over 300 

 teeth, were found, resembling in many respects milk-teeth ;" 

 and to another tumour, " full of hair which had grown and 

 " been shed from one little spot of skin not bigger than the tip 

 " of my little finger. The amount of hair in the sac, had it 

 " grown from a similarly sized area of the scalp, would have 

 '• taken almost a lifetime to grow and be shed." 



Whether each of the innumerable autonomous elements of 

 the body is a cell or the modified product of a cell, is a more 

 doubtful question, even if so wide a definition be given to the 

 term, as to include cell-like bodies without walls and without 

 nuclei. 40 The doctrine of omnis celhda e cellidd is admitted 

 for plants, and widely prevails with respect to animals. 41 

 Thus Yirchow, the great supporter of the cellular theory, 

 whilst allowing that difficulties exist, maintains that every 

 atom of tissue is derived from cells, and these from pre- 

 existing cells, and these primarily from the egg, which he 

 regards as a great cell. That cells, still retaining the same 

 nature, increase by self-division or proliferation, is admitted 

 by every one. But when an organism undergoes great 

 changes of structure during development, the cells, which at 

 each stage are supposed to be directly derived from previously 

 existing cells, must likewise be greatly changed in nature ; 

 this change is attributed by the supporters of the cellular 

 doctrine to some inherent power which the cells possess, 

 and not to any external agency. Others maintain that cells 

 and tissues of all kinds may be formed, independently of pre- 

 existing cells, from plastic lymph or blastema. "Whichever 

 view may be correct, every one admits that the body consists 

 of a multitude of organic units, all of which possess their 

 own proper attributes, and are to a certain extent independent 

 of all others. Hence it will be convenient to use indifferently 

 the terms cells or organic units, or simply units. 



Variability and Inheritance. — We have seen in the twenty- 

 second chapter that variability is not a principle co-ordinate 

 with life or reproduction, but results from special causes, 



*' For the most recent classification 41 Dr. W*. Turner. 'The Present 



of cells, see Ernst Hackel's ' Generelle Aspect of Cellular Pathology,' ' Edin« 

 Morpholog.,' Baud ii., 186(3, s. 275. burgh Medical Journal.' April, 18fi3. 



