Chap. XXYII. OF PANGENESIS. 377 



continue multiplying without being modified by their union 

 with free gemmules of any kind, is probable from such cases 

 as that of the spur of a cock which grew to an enormous size 

 when grafted into the ear of an ox. How far units are 

 modified during their normal growth by absorbing peculiar 

 nutriment from the surrounding tissues, independently of 

 their union with gemmules of a distinct nature, is another 

 doubtful point. 51 We shall appreciate this difficulty by 

 calling to mind what complex yet symmetrical growths the 

 cells of plants yield when inoculated by the poison of a 

 gall-insect. With animals various polypoid excrescences and 

 tumours are generally admitted 52 to be the direct product, 

 through proliferation, of normal cells which have become 

 abnormal. In the regular growth and repair of bones, the 

 tissues undergo, as Virchow remarks, 53 a whole series of 

 permutations and substitutions. " The cartilage cells may 

 " be converted by a direct transformation into marrow-cells, 

 " and continue as such ; or they may first be converted into 

 " osseous and then into medullary tissue ; or lastly, they may 

 " first be converted into marrow and then into bone. So 

 " variable are the permutations of these tissues, in themselves 

 " so nearly allied, and yet in their external appearance so 

 " completely distinct." But as these tissues thus change 

 their nature at any age, without any obvious change in their 

 nutrition, we must suppose in accordance with our hypothesis 

 that gemmules derived from one kind of tissue combine 

 with the cells of another kind, and cause the successive 

 modifications. 



We have good reason to believe that several gemmules are 

 requisite for the development of one and the same unit or 

 cell; for we cannot otherwise understand the insufficiency 

 of a single or even of two or three pollen-grains or sper- 

 matozoa. But we are far from knowing whether the gemmules 

 of all the units are free and separate from one another, or 

 whether some are from the first united into small aggregates. 



51 Dr. Ross refers to this subject trans, by Dr. Chance, 1860, pp. GO, 

 in his ' Graft Theory of Disease,' 1872, 162, 245, 441, 454. 



p. 53. 53 Ibid., pp. 412-426. 



52 Virchow, ' Cellular Pathology,' 



