DEVELOPMENT 



415 



earthy matter (chiefly phosphate of lime) is deposited 

 between the cells. The primary bone thus formed is 

 compact : true osseous tissue, with canaliculi, laminae, 

 and Haversian canals, is the result of subsequent ab- 

 sorption. 167 Certain bones, as those of the face and 

 cranium, are not preceded by cartilage, but by connec- 

 tive tissue ; these are called membrane bones. Ossi- 

 fication, or bone making, begins at numerous distinct 

 points, called centers of ossification ; and, theoretically, 

 every center stands for a bone, so that there are as many 

 bones in a skeleton as centers of ossification. But the 

 actual number in the adult animal is much smaller, as 

 many of the centers coalesce. 168 The development of 

 the backbone is not from the head or from the tail, but 

 from a central point midway between ; there the first 

 vertebrae appear, and from there they multiply forward 

 and backward. 



The limbs appear as buds on the sides of the body ; 

 these lengthen and expand so as to resemble paddles 

 the wings and legs looking precisely alike ; and, finally, 

 they are divided each into three segments, the last one 

 subdividing into digits. The feathers are developed 

 from the outside cells of the epidermis : first, a horny 

 cone is formed, which elongates and spreads out into a 

 vane, and this splits up into barbs and barbules. 



The muscle fibers are formed either by the growth in 

 length of a single cell, or by the coalescence of a row of 

 cells ; the cell wall thus produces a long tube the sar- 

 colemma of a fiber and the granular contents arrange 

 themselves into linear series, to make fibrillae. 



Nervous tissue is derived from the multiplication and 

 union of embryo cells. The white fibers at first resem- 

 ble the gray. The brain and spinal marrow are devel- 

 oped from the epiblastic lining of the medullary furrow. 

 Soon the brain, by two constrictions, divides into fore 



