Nov., 1889. 
THEORIES OF HEREDITY. 
251 
tlie process of fertilisation. The germ-plasm is in reality 
only found in the nucleus of the cell, but this is omitted 
from the diagram for the sake of simplicity. The germ-plasm 
is supposed to be represented by the dots in the circle A. 
The second stage (II.) is produced by the division of the 
ovum into two cells (13 and C), each of which will give rise, 
by subsequent division, to one of two great classes of somatic 
cells. This is no theory : it rests upon many observations. 
When the subsequent history of each cell has been watched 
in certain animals, it has been found that different classes 
of cells have been produced. Hence, by the division of the 
ovum A, two cells are produced which are unlike each other 
and unlike the ovum ; the process is not merely one of 
halving, but of differentiation. This is important to bear 
in mind, because halving sometimes occurs and leads to 
very different results, as will be seen below. But Weismann 
also supposes that a minute part of the germ-plasm in the 
ovum escapes the process of differentiation, and is carried 
unchanged in one of the cells of the second stage. It is 
represented as the smaller circle A, enclosed within C. 
Not only are cells separated very early to form the great 
classes of somatic cells which will afterwards appear, but the 
sides of the body, and its hind and front ends, are also soon 
determined. It appears that, in some animals, the great groups 
of ceils are determined by the first division ; in others the 
right and left sides, or front and hind ends of the body ; while 
the cells giving rise to the chief groups on each side would 
then be separated at some later division. This is not theory 
but fact; for Boux has recently shown that, if one of the 
products of the first division of the egg of a frog be destroyed 
with a hot needle, development is not necessarily arrested, 
but, when it proceeds, leads to the formation of an embryo 
from which either the right or the left side is absent. When 
o 
the first division takes place in another direction, either the 
hind or front half was absent from the embryo which was 
afterwards produced. After the next division, when four 
cells were present, destruction of one produced an embryo 
from which one-fourth was absent.* 
Hence either the great groups of cells or the sides or ends 
of the body may be predetermined in the first division of the 
egg ; and we may feel sure that, although the order varies in 
different animals, both results occur very early. In Diagram 
* My attention was first directed to these interesting experiments 
by Professor Wiudle’s paper in the “ Journal of Anatomy and Physi¬ 
ology,” Vol. xxiii., p. 393. 
