o2 
POPULAR SCIENCE REVIEW. 
thus set at liberty wandered about until they once more com- 
bined with a plant or animal — here with a monad, there with a 
(juadruped. The materia vitce diffusa of John Hunter was 
somethinor similar. 
Jn the present day physiologists, with the assistance of those 
powerful microscopes which opticians have placed in their hands, 
have traced the changes which ova and seeds undergo during 
tlieir development in a vast number of animals and of plants. 
In this way it has been established that some forms of animal 
life may be propagated without generation by parents. Such 
interruption in descent Owen has called parthenogenesis, from 
nrap6£vsla, virginity. Thus, two winged insects will produce an 
animal without wings, from which ten or twelve generations of 
individuals may be derived without a fresh act of conception, 
until the last in the chain gives forth another winged insect, 
when the process is repeated — as in the case of the aphis. 
Lastly it has been shown that the very lowest forms both of 
vegetable and animal life cannot be traced back to spores or 
ova. The law of descent, therefore, from parents, homogenesis, 
which we recognise in the higher organisms, changes as we de- 
scend in the scale, first to parthenogenesis, where this direct 
descent is broken, and ultimately to heterogenesis in which it is 
lost. It is to the last of these processes attention is directed 
in the present article. 
On making a cold or hot infusion of any vegetable or animal 
substance, covering the vessel with a piece of paper so as to 
exclude the dust, and then watching it every twelve hours, the 
first change visible to the eye is a slight opalescence, and the 
formation of a thin scum or pellicle that floats upon the surface. 
This appears at times, varying from a few hours to several 
days, according to the temperature of the atmosphere or the 
nature of the infusion. On examining the pellicle or film 
under hio-h magnifying powers, it is seen to be composed of a 
mass of minute molecules, varying in size from the smallest 
visil)le point to that of one thirty-thousandth of an inch in 
diameter, d'hese molecules are closely aggregated together, 
and must exist in incalculable numbers. They constitute the 
primordial mucous layer of Burdach,* and the proligerous 
])ollicle of Pouchet.t The same pellicle, examined six hours 
later, shows the molecules to be somewhat enlarged, and these 
se])arated by the pressure of the upper glass are already seen 
here and there to be strongly adhering together in twos and 
f.mrs, so as to form a little chain. Many twos, also, have ap- 
}»arently melted together so as to form a short staff or filament 
— bode) lain (fig. 1, />). Twelve hours after this, it maybe 
• “ I’hysiologiR, par Jourdau,*’ tome ii. p. 12d. 
t ‘‘ lldteiojunie/’ p. 
