SOME PROBLEMS OF REPRODUCTION. 79 
Postscript.—In writing the foregoing essay, and even in 
seeing it through the press, I omitted to state one cardinal 
point, really underlying the whole theory of gametogeny now 
proposed; but the necessity of expressing it in set terms 
appeared as soon as I had to consider the best mode of pre- 
senting a concise account of my views to an audience of the 
British Association in Cardiff (August 22nd), 1891. Two 
distinct modes of fission occur in relation to the growth of the 
organism in Protozoa and Protophytes : in the first, after each 
division the daughter-cells grow to the size of the parent 
(more or less) before dividing in turn; in the second, the 
intervals of growth are suppressed, and a series of successive 
fissions takes place, resulting in a brood of small individuals 
(“ swarmers,” ‘‘ zoospores,” &c.). We call this second type of 
fission ‘‘ brood-formation,” the resulting individuals ‘ brood- 
cells.” Necessary, like facultative, gametes are essentially, 
in origin at least, modified brood-cells. Hence, when the 
ancestral development is not lost, gametes will always be 
produced by brood-formation, while tissue-cells (ex- 
cept in the earlier embryonic state) are formed by the first 
mode of fission. 
September, 1891. 
