Heredity and its Bearings on Atavism. 141 
also bear in mind the anatomical. arrangement of groups 
of cells differing in their functions, ey., are we dealing 
with a concentric arrangement as in the Hydra, in which 
the ectoderm, the mesogloea, and the endoderm are placed 
one within the other; or have we a bilateral, a dorso-ventral, 
or an antero-posterior grouping ? 
Apart from this primary arrangement of tissues, we may 
have a secondary organisation, for we may find the same 
anatomical distribution of all the organs in different parts 
of the same individual, eg., in the Starfish, with its 
pentaradial system, each arm with its corresponding part 
of the disc resembles its neighbour; in plants, e.g., Begonia, 
the tissue of each leaf is divided into a dermatogen, a 
periblem, and a plerome. 
Finally, some of the organs of a multicellular being may 
occur in multiple number, while other organs are only found 
singly, thus the segmental organs of a worm and the eyes of 
vertebrates are bilateral, while there is, apparently, only one 
ventral gangliated chain in the former, only one heart in 
the latter. 
The bearing of these facts on Heredity will become 
evident immediately. 
If a Hydra, a Starfish, or a Begonia plant be individuals, 
how is it that parts of these organisms may give rise, under 
certain conditions, to new and entire individuals? for if the 
arm of a Starfish, with its corresponding portion of disc, be 
cut off, this arm is found to have the power of developing 
into a complete individual. If a Hydra be divided into 
several fragments, each of these may give rise to a complete 
Hydra; again, portions of Begonia leaves are daily used for 
propagating this plant. 
An explanation of this propagation of individuals must 
account for two facts, namely, firstly, how is it that a portion 
of a mother-individuum can live after being separated 
artificially, and secondly, why should the fragment assume 
the shape of the mother-individuum, eg., why can an arm 
of a Starfish live by itself, and why does it give rise to four 
other arms, thus re-establishing the pentaradial form ? 
We saw above that each arm of the Starfish contains all 
