440 
effects from apparently the same cause. Neither can it be attri- 
buted to any direct influence of the cell or of its nucleus, the germinal 
vesicle. For example, an egg is fully maturated in the female 
organs of generation, and would prove abortive if a spermatozoid did 
not find its way through the zona-pellucida and get among the mole- 
cules of the yoke. As soon as it does so, the apparently purposeless 
Brunonian movements receive a new impulse and direction. Both 
spermatozoid and germinal vesicle are dissolved among them, and 
that wonderful phenomenon of the division of the yoke takes place, 
not by cleavage or other action of the cell wall or nucleus, but by 
the separation of the mass into two masses instead of one. This 
was compared to what is observable in a dense crowd of men, called 
upon to pass over to the right or left hand in order to settle any 
disputed question by a majority. At first unusual confusion is com- 
municated to the whole ; some hurry in one direction, others in 
another ; but after a time there is seen at the margins, where the 
crowd is least dense, a clear space, which gradually approaches the 
centre, and at length bisecting the whole, produces a complete segrega- 
tion of the crowd into two portions. So with the molecules of the yolk 
in the egg after impregnation ; their movements are directed by con- 
ditions which did not previously exist, and a stimulus is imparted to 
them which causes the peculiar result. It is the division and subdi- 
vision of the yolk, wholly or in part, which produces the germinal mass 
out of which the embryo is formed, and this not by any direct influ- 
ence of the cell or nucleus, but in consequence of a power inherent 
in the molecules themselves, which was communicated to them for 
a specific purpose. 
4:th, The peculiar movements so well described by Briicke, Yon 
Wittich, Harless, and especially by Lister, in the pigment cells of 
the frog’s skin;* and which occasion the sudden change of colour in 
the cameleon, in fishes, and numerous other animals. The black 
pigment molecules may be diffused throughout the cell or concen- 
trated in a mass, and all kinds of intermediate gradations may exist 
between diffusion and concentration. The change in colour is 
owing to these alterations in the molecules, the tint being light when 
they are concentrated, and dark when they are diffused. Mr Lister 
ascertained by experiment that their concentration is caused by ex- 
* On the Cutaneous Pigmentary System of the Frog. — Philosophical Trans- 
actions, 1858. 
