330 PROF. EHRENBERG ON ANIMALS OF THE CHALK 
cases where accurate examination could be made, multiplication 
by impregnated seed and eggs has been recognised as giving rise 
to a certain fluctuation in the types of form of the posterity, by 
which the characters of the species frequently pass into each 
other, so spontaneous fission as well as gemmation, are modes 
of increase co-existing with oviparous generation, which do not 
favour this fluctuation of the types of form, but rather necessitate 
a certain constancy and stability, nay, similarity of the posterity 
in relation to their progenitors. In this, gemmation differs from 
spontaneous fission in degree. Gemmation, which is chiefly 
peculiar to vegetable organisms, yet also occurs frequently in 
minute animals, perpetuates distinctly the peculiarity of the pa- 
rent body, but soon affords to the posterity a perfectly indepen- 
dent further development, which, merely through the elongated 
immediate substance and community of circulation of the parent 
body, remains for a time in closer connexion with the individu- 
ality of the same than the egg, and needs no impregnation. Self- 
division, on the other hand, restricts the free individual deve- 
lopment of the posterity in the highest degree. The animals 
individualizing themselves, i.e. becoming organically indepen- 
dent, are, up to the half, the parent body itself, whose other half 
then constantly regenerates itself out of each individual. This 
mode of increase appears at first sight similar to the “laying 
down ” of plants in horticulture, but physiologically it is some- 
thing quite different. The layer or disconnected shoot, or even 
the stem divided into several parts for separate planting, is indeed 
the divided parent body itself, and one expects with confidence 
that it will have like development, and bear like blossoms and 
fruit. Nevertheless, the stem or twig is no demonstrated nor de- 
monstrable individual; and in these cases we have rather under- 
taken a process which is similar to the division of a coral stem, by 
which the individual is not necessarily altered, nay, not even ne- 
cessarily touched, merely the social form has been disturbed and 
changed in its combination. But it is expressly the organic indi- 
vidual which by self-division of the animal is essentially and ne- 
cessarily altered, as it divides itself only in such manner that it se- 
parates its perfect organism self-actively into two halves, andcloses 
anew [#. e. heals the wound of division], converting each half, by 
means of an activity similar to the organic regenerative force, into’ 
a separate organism, frequently capable of perfect independency. 
On perfect self-division two individual bodies originate from 
one individual body, each of which two possesses, and actually 
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