Heterotype Karyokinesis and its Significance. 31 
always has double the number of chromosomes, and shews typical 
homotype divisions. 
This fundamental cytological resemblance between the tissue 
producing sexual cells and that of malignant tumours no doubt 
implies some deep-seated physiological similarity. It is suggested 
by Professor Farmer and his collaborators that the immediate 
cause of malignancy is the transformation of the type of cell- 
development represented by the change from homotype to hetero¬ 
type, while the remote cause, though, at present obscure, must lie in 
the action of physiological stimuli, perhaps of the nature of con¬ 
tinuous irritation, which, in some cases at least is known to be 
connected with the origin of cancer. The normal cycle of develop¬ 
ment in the life of an organism is probably determined by a definite 
succession of automatically produced physiological stimuli, and this 
succession is interrupted, so to speak, at two points in the 1 ife- 
cycle, corresponding with the reduction of chromosomes and with 
the process of fertilisation (at which the number of chromosomes is 
doubled) respectively. Each of these interruptions forms a new 
starting point and is succeeded by a new development, corresponding 
no doubt with the inauguration of a fresh succession of stimuli. It 
is now probable that the new departure occurring in a malignant 
tumour is of fundamentally the same nature as that which marks 
the origin of the sexual cells or of the gametophyte in plants. It 
is significant, as the authors point out, that in the higher plants the 
gametophyte is more or less parasitic upon the parent organism, 
just as the malignant tumour is. It must however be remembered 
that in the Archegoniates it is the sporophyte, the product of fer¬ 
tilisation which is parasitic on the gametophyte, as the embryo of 
many animals is on the parent. We may therefore, perhaps, take the 
more general ground that the change in the course of development, 
the inauguration of the new series of automatic stimuli, both at 
chromosome-reduction and at fertilisation, is equivalent to the 
establishment of independence in the new organism which may 
lead to a parasitism on the preceding generation, more or less con¬ 
trolled, in the case of the normal life-cycle by the necessity of 
adaptation to the continuation of the species, but in the case of 
malignant growths, of course, quite uncontrolled in such a way. 
We have in the present notice, dwelt wholly on the general 
biological aspects of the new discovery. With its pathological, 
clinical and possibly therapeutic implications we cannot, of course, 
deal. It seems however to negative the theories of a specific 
