=Significance of the Chromosomes.--=The question naturally arises as to what is the significance of the chromosomes. Why is the accurate adjustment which we have noted for their division necessary? The very existence of an elaborate mechanism so admirably adapted to their precise halving, predisposes one toward the belief that the chromosomes have an important function which necessitates the retention of their individuality and their equal division. Many biologists accept this along with other evidence as indicating that in chromatin we have a substance which is not the same throughout, that different regions of the same chromosome have different physiological values. When the cell prepares for divisions, the granules, as we have seen, arrange themselves serially into a definite number of strands which we have termed chromosomes. Judging from all available evidence, the granules are self-propagating units; that is, they can grow and reproduce themselves. So that what really happens in mitosis in the splitting of the chromosomes is a precise halving of the series of individual granules of which each chromosome is constituted, or in other words each granule has reproduced itself. Thus each of the two daughter cells presumably gets a sample of every kind of chromosomal particle, hence, the two cells are qualitatively alike. To use a homely illustration we may picture the individual chromosomes to ourselves as so many separate trains of freight cars, each car of which is loaded with different merchandise. Now, if every one of the trains could split along its entire length and the resulting halves each grow into a train similar to the original, so that instead of one there would exist two identical trains, we should have a phenomenon analogous to that of a dividing chromosome. =Cleavage of the Egg.--=It is through a series of such divisions as these that the zygote or fertilized egg-cell builds up the tissues and organs of the new organism. The process is technically spoken of as _cleavage_. Cleavage generally begins very shortly after fertilization. The fertile egg-cell divides into two, the resulting cells divide again and thus the process continues, with an ever-increasing number of cells. =Chief Processes Operative in Building the Body.--=Although of much interest, space will not permit of a discussion in detail of the building up of the special organs and tissues of the body. It must suffice merely to mention the four chief processes which are operative. These are, (1) infoldings and outfoldings of the various cell complexes; (2) multiplication of the component cells; (3) special changes (_histological differentiation_) in groups of cells; and (4) occasionally resorption of certain areas of parts.