600 LAWS OF ORGANIC DEVELOPMENT. 
so very different from the segments in other parts of the column, 
the question rests entirely on the definition of a vertebra. If a 
vertebra be a segment of the skeleton, of course the skull is com- 
posed of vertebre ; if not, then the cranium may be said to be 
formed of ‘‘ sclerotomes,” or some other name may be used. Cer- 
tain it is, however, that the parts of the segments of the cranium 
may be now more or less completely parallelised or homologised 
. with each other, and that as we descend the scale of vertebrated 
animals, the resemblance of these segments to vertebree increases, 
and the constituent segments of each become more similar. In 
the types where the greatest resemblance is seen, segmentation of 
either is incomplete, for they retain the original cartilaginous 
basis. Other animals which present cavities or parts ôf a solid 
support are still more easily reduced to a simple basis of segments, 
arranged either longitudinally (worm) or centrifugally (star-fish, 
etc.) 
Each segment — and this term includes not only the parts of a 
complex whole but parts always subdivided, as the jaw of a whale 
or the sac-body of a mollusk,—is constructed, as is well known 
y cell division. In the growing foetus the first cell divides its 
nucleus and then its whole outline, and this process repeate 
millions of times produces, according to the cell theory, all the 
tissues of the animal organism or their bases from first to last. 
That the ultimate or histological elements of all organs are pro- 
duced originally by repetitive growth of simple, nucleated cells 
with various modifications of exactitude of repetition in the more 
complex, is taught by the cell theory. The formation of some of 
the tissues is as follows 
First Change— Formation of simple nucleated cells from homo- 
geneous protoplasm or the cytoblastema. 
cond— Formation of new cells by division of body wes nu- 
cleus of the old. 
ird— Formation of tissues by accumulation of cells with or 
without addition of intercellular cytoblastem 
. In connective tissue by slight ile of cells and addi- 
tion of eytoblastema. 
n blood, by addition of fluid cytoblastema (fibrin) to free 
dete pea corpuscles), which in higher animals (vertebrates) 
develop into blood-corpuscles by loss of membrane, and by ce 
development of nucleus. 
