THE ANIMAL ANCESTRY OF MAN 63 



and vocal cords, if the initial steps in these arrangements 

 had not been taken by our eadiest chordate ancestors. 



Man is also indebted to such very early chordates for 

 another "invention" of the greatest importance, namely 

 the bone-cell. Apparently originating in the deeper layers 

 of the skin, the bone-cells later invaded the connective- 

 tissue partitions between the muscle segments and ultimately 

 gave rise to the internal skeleton. Perhaps the physiologists 

 may be able to find out why the calcium phosphate and cal- 

 cium carbonate w^re deposited in the Haversian system of 

 capillaries by these pecuhar cells, instead of being cast off 

 by the excretory system. 



Our catalogue of debts to the ostracoderms, or to some of 

 their contemporaries, is further increased by the fact that 

 they seem to have been the first of the chordate series to 

 develop a "head shield," or bony mask covering the entire 

 gill chamber and inner brain-case. In the more typical 

 ostracoderms the surface head shield appears to have been 

 all of one piece; but in certain of the anaspid ostracoderms 

 the head was covered by small dermal plates, somewhat 

 after the fashion that was adopted by our own ancestors. 



All recent evidence tends to support the view not only 

 that the modern cyclostomes are the, in some respects 

 degenerate, descendants of certain of the ostracoderms 

 but that Amphioxus represents a still further degenerate 

 derivative of the same stock. 



OUR FOREBEARS ATTAIN THE GRADE OF VERTEBRATES 



In the first chordates the elastic axial tube, or notochord, 

 as seen in Amphioxus, is continuous and unsegmented; but 

 later when the vertebrate grade of organization was attained, 

 rods and blocks of skeletal tissue began to be secreted under 

 the influence of the muscle segments, and as these blocks 

 increased in importance they gradually replaced the primary 

 backbone, or notochord, and gave rise to the secondary 

 backbone or vertebral column. In the ostracoderms appar- 

 ently only a notochord was present; in the arthrodires (a 

 group of extinct fishes of the Devonian period) the rods 

 above and below the notochord had become hard enough 

 in the tail region to leave their imprints in the surrounding 



