2 Allgemeines. 



as well as those in the cell-generations oi the reproductive tissues that 

 have preceded it are as follows: 



1. The period of rest and growth. 



2. The chromosomes when they are formed from the resting nucleus 

 are present in only half the number of those occurring in the rest of the 

 dividing nuclei of the organism. 



3. The forms exhibited by these chromosomes are strikingly diffe- 

 rent from those of other nuclei. They produce figures resembling loops. 

 rings, aggregations of four heads, and so on. 



4. Their divisions on the spindle is transverse and not longitudinal. 



It will be thus seen that this heterotype mitosis is an easily recog- 

 nised phase in the history of the development of the sexual cells. 



It is a fact of the highest importance that when once the hetero- 

 type division has supervened, all the descendants of that cell retain the 

 reduced number of chromosomes in normal cases. The cycle of these 

 cell-generations, the nuclei of which only form half the somatic number 

 of chromosomes, normally closes with the formation of the definite sexual 

 cells. It is on the fusion of two of these (ovum and Spermatozoon) that 

 the double or somatic number is restored, and this number is characte- 

 ristic of the fertilised egg, and of all the cells to which it gives rise 

 until the heterotype mitosis again supervenes in the reproductive tissues. 

 Xow after the Intervention of the heterotype division, the cell in which 

 it has occured may, after one further division, at once give birth to the 

 four sexual cells, as in the higher animals, or, on the other hand, a 

 varying number of cell-generations may be intercalated before the final 

 differentiation of the sexual elements. This occurs in the majority of 

 plants. It is in these latter that the commonly parasitic character of the 

 organism thus arising is specially, though not exclusively, apparent. 

 Thus, the embryo sac of many flowering plants exerts a destructive in- 

 fluence on the cells of the soma adjacent to it. This property is not, 

 however, by any means exclusively confined to the post-heterotype for- 

 mation (the gametophyte of the plant), and we do not wish to lay dis- 

 tinctive weight upon it. In the lower plants the bulk of the body is 

 composed of cells with reduced nuclei, and the alternate stage in the life 

 cycle, originating in the fertilised egg, is the predatory structure. What 

 seems to emerge from a general consideration of the whole ränge of facts 

 is this: that in the higher animals and plants the post-heterotype tissue, 

 with its own independence of Organisation, does behave towards the sur- 

 rounding tissues of the parental individual as a neoplasm. So far as 

 the parent is concerned, the new growth might be described as a patho- 

 logical one, did it not form a normal stage of the life-history of the 

 species. 



We have said that the cells from which the heterotypically dividing 

 elements will finally arise can often be distinguished from those cells 

 which will not produce such elements. In the testis of a mammal or in 

 the sporogenous tissue of a stamen we recognise with ease and certainty 

 the existence of these cells. They continue to multiply, and though 

 differing from the adjacent cells in many respects, they continue to re- 

 semble them in their mode of nuclear division until they pass severally 

 into the peculiar State of growth that ushers in the heterotype division. 



In their studies of abnormal growths occurring on ferns_, the authors 

 were Struck by certain features presented by the proliferating tissues that 

 are formed during apogamy and apospory, and have thus been led to 

 make a systematic investigation of the cytological features presented by 

 malignant growths in man. 



This has resulted in the recognition of the existence of a surprising 

 degree of similarity between the phases that characteristically recur in 

 such tissues and those transformations of somatic cells into reproductive 

 tissues in general. 



Thus in a typical example of rapidly growing epithelioma it is seen 

 that in the early stages of the proliferation of the Malpighian layer, the 

 cells of the invading tissue at first pass through a cycle of somatic 



