314 BIOLOGY AND ITS MAKERS 



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 



