445 
from being exclusively seated in cells, is also intimately associated 
with the elementary molecules of the organism. 
III. This leads me, in the third place, to an enunciation of the mo- 
lecular law of growth, which a study of the numerous facts previously 
referred to has induced me to frame, viz. : — That the development 
and growth of organic tissues is primarily owing to the successive 
formation of histogenetic and histolytic molecules. We have already 
seen that development and growth in animals originate in the 
molecules of the yolk of the egg, or of a germinal molecular mass 
formed from it. The author referred to numerous careful researches 
recognised by scientific men as giving a correct account of the de- 
velopment of various animals and textures. From these it would 
appear that the first form was molecular; that the molecules united 
to produce nuclei and cells ; that these became disintegrated to pro- 
duce a secondary mass of molecules ; that these again united to form 
secondary nuclei and cells ; and that the same process was repeated 
more or less often in various developments, until the animal or 
tissue was formed. This constituted the successive histogenetic and 
histolytic molecules observable in the process of growth, — the former 
building up, to a certain extent, and the product disintegrating to 
produce the latter, which after a time, again, re-arranged itself and 
became histogenetic to form cells or tissues, which in their turn broke 
down and became histolytic. In short, not only development, but 
growth and secretion, absorption and excretion, were only different 
names given to histogenetic and histolytic processes, and that these 
were brought about by formative and disintegrative molecules. As 
illustrations of this law, the author minutely followed the develop- 
ment of Ascarix mystax , as described by Nelson,* and of the pro- 
cess of nutrition in the human body. 
In this, and a vast number of similar observations, it must be evi- 
dent that a certain series of molecular transformations is necessary 
for the one which follows it. Thereby is produced a continual ela- 
boration of matter, — a constant chemical and morphological series of 
changes, — the exact number and order of which, in the production of 
organic forms, only requires time and perseverance to discover. 
Doubtless various conditions, dynamical, chemical, and vital, must 
co-operate in producing the result, and they must all influence mole- 
* Philosophical Transactions, 1850, plates xxviii., xxix., figs. 59. G8, 70, 78. 
