1906.] Observations on the Life- History of Leucocytes. 



57 



figures, others are found exactly similar to those that I have described as 

 second maiotic and post-maiotic in leucocytes (see figs. 27 and 28). In 

 Triton testes it is possible to trace stages between the cells that have always - 

 been regarded as, and probably are, connective tissue cells and the cells that 

 apparently perform the same function as the foot-cells of mammalian testes. 

 In Triton also, during the earlier stages of the maiotic phase, the cells that 

 are destined to become foot-cells are similar to certain leucocytes in the same 

 animal. Accepting these observations as correct, and regarding the pockets 

 of the amphibian testis as being directly comparable with the tubules of the 

 mammalian testis, 1 am forced to the conclusion that either the leucocytes 

 themselves or their immediate ancestors may give rise to connective tissue, 

 the former probably being what really happens. 



While, as has already been said, there are remarkable points of similarity 

 between the life-histories of the leucocytes and those cells in plants which, 

 though reduced, never become converted into sexual elements, it is also 

 evident that there are some important points of difference. In plants, both 

 the cells just referred to and those which are converted into definite sex cells 

 have commenced the maiotic phase at the same time, and their immediate 

 ancestry is common to both. In the case of leucocytes and certain connective 

 tissues it is not at present demonstrable that anything of the kind happens. 

 It may be that our present conception of what constitutes the whole of the 

 maiotic phase, in animals at any rate, is too limited, and that the develop- 

 ment of the mesoblast is in some way involved in its earliest stages. In this 

 connection the fact that in some plants reduced cells may be differentiated 

 into tissues that are somatic in characters and function is extremely 

 suggestive, as are also the observations of Loeb upon the segmentations and 

 the production of embryos in unfertilised eggs. 



It does not seem out of place to mention here the bearing that these obser- 

 vations have upon what happens in cancer. As has been shown elsewhere* 

 one of the earliest phenomena observed in the development of cancer is the 

 fusion of a leucocyte with a tissue cell and the subsequent division of the 

 cell resulting from the fusion into two daughter cells, each possessing 

 chromatic elements derived partly from the leucocyte and partly from the 

 tissue cell. That in the subsequent generations of the cells produced from 

 this fusion the characters of both ancestors should appear is exactly what 

 would be expected. Among the cells of malignant growths all the forms of 

 division here recorded as occurring among leucocytes and their immediate 

 ancestors are to be found. As has also been stated before, some of the 



* Farmer, Moore arid "Walker, " On the Behaviour of Leucocytes in Malignant Growths," 

 ' Trans, of the Path. Soc. London,' vol. 56, Part III, 1905. 



