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 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 com- 
plexes; (2) multiplication of the component cells; (3) special changes 
(histological differentiation) 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, 
