BENNETT^ OX THE MOLECULAR THEORY. 47 



that the results are purely physical^ because in different ova 

 we see such widely Aarying effects from apparently tlie same 

 cause. Neither can it be attributed 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 molecules of the yoke. As soon as it does so, the ap- 

 parently purposeless Bnmonian movements receive a new 

 impulse and direction. Both spermatozoid and germinal 

 vesicle are dissolved among them, and that wonderful phe- 

 nomenon 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 communicated 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 segregation 

 of the crowd into two portions. So with the molecules of 

 the yolk in the egg after impregnation ; their movements 

 are directed by conditions which did not previously exist, and 

 a stimulus is imparted to them which causes the peculiar 

 result. It is the division and subdivision 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 influence 

 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. 



4th. The peculiar movements so well described by Briicke, 

 Von Wittich, Harless, and especially by Lister, in the pig- 

 ment-cells of the frog's skin." and which occasion the 

 sudden change of colour in the chameleon, in fishes, and 

 numerous other animals. The black pigment-molecules may 

 be diff'used throughout the cell or concentrated 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 exposure to the light, by death of the animal, 



* " On the Cutaneous Pigmentary System of the Prog." ' Philosopliical 

 Transactions/ 1858, 



