HEREDITY AND GERMINAL CONTINUITY = 313 
their union in fertilization results in the restoration of the 
original number of chromosomes. ‘The fertilized ovum is 
the starting-point of a new organism, and from the method 
of its fertilization it appears that the parental qualities are 
passed along to the cells of every tissue. 
The complex mechanism exhibited in the nucleus during 
segmentation is very wonderful. The fertilized ovum begins 
to divide, the nucleus passing through a series of complicated 
changes whereby its chromosomes undergo a_ lengthwise 
division—a division that secures an equable partition of the 
substance of which they are composed. With each successive 
division, this complicated process is repeated, and the many 
cells, arising from continued segmentation of the original cell, 
contain nuclei in which are embedded descendants of the 
chromosomes in unbroken succession. Moreover, since these 
chromosomes are bi-parental, we can readily understand that 
every cell in the body carries both maternal and paternal 
qualities. 
The careful analysis of the various changes within the 
nuclei of the egg proves to be the key to some of the central 
questions of heredity. We see the force of the point which 
was made in a previous chapter, that inheritance is in the 
long run a cellular study, and we see in a new light the im- 
portance of the doctrine of germinal continuity. This con- 
ception, in fact, elucidates the general problem of inheritance 
in a way in which it has never been elucidated by any other 
means. 
For some time the attention of investigators was concen- 
trated upon the nucleus and the chromosomes, but it is now 
necessary to admit that the basis of some structures is dis- 
coverable within the cytoplasm that surrounds the nucleus. 
Experimental observations (Conklin, Lillie, Wilson) have 
shown the existence of particular areas within the apparently 
simple substance of the egg, areas which are definitely related 
