294 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



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; this is, they can 

 grow and reproduce themselves. So that what really happens in mito- 

 sis 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 dividmg 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, (i) infoldings and outfoldings of the various cell com- 

 plexes; (2) multiplication of the component cells; (3) special changes 

 {histological diferentiation) in groups of cells; and (4) occasionally 

 resorption of certain areas of parts. 



The origin of the new germ-cells. — On account of the unusual 

 importance from the standpoint of inheritance, which attaches to the 

 germ-cells, a final word must be said about their origin in the embryo. 

 While the evidence is conflicting in some cases, in others it has been 

 well established that the germ-cells are set apart very early from the 

 cells which are to differentiate into the ordinary body tissues. Fig. 45, 

 A, shows a section through the eight-celled stage of Miastor, a fly. 



