4ol 



CHORDATA 



plate which has arisen from a fusion of dermal ossifications, liecomes trans- 

 ferred to a deeper position, and contriluites to the completion of the axial 

 skeleton. From what was said aliove aliout the relations of dermal ai\d 

 true teetli it is readily seen that the lining of the mouth cavity is a 

 source of meml irane 1 'ones. 



In descrining the axial skeleton, the notochord comes first. Tliis has 

 already keen mejitionetl in connexion «'iih lower CMiordates. It persists 

 in the cyclostomes, kul from tlu'm up\\'ard.s it is gradually re].)laced \>y the 

 vertebne arising around it. It is of entodermal origin (^fig. o). arising as a 

 longitudinal kand of the epithelium of the 

 archenleron (I. (if), and, kecoming cut off, 

 comes to lie in the long axis of the kodv ke- 

 tween digestive tract and ner\"ous system (7/, 

 III) ; here it forms a cyclindrical rod consisting 

 of a connecti\'e tissue which, as ahwulv said, 

 resembles plant tissues kecause of the vesicular 

 nature of its cells (hg. 30). 



In transverse section (tig. 511) tire noto- 

 chord is surrounded \>\ three la\'ers. internallv 

 ky a fikrous noloclicrdal sltcalii, then a)i elastic 

 layer (not alwa}'s present^ the dasliui cxlcnia. 

 so called kecause an elastica interna sometimes 

 occurs inside the nolochordal sheath: autl 

 lastly a skeletogenous la}'er (^\S"). This last 

 is a connective-tissue la\'er and is therefore 

 connected with the other conneclixe tissues 

 which surround muscles, nerxes, etc. It de- 

 serves special mention kecauseinit thecartilages 

 and kones arise from which the X'crteknr and 

 skull are formed. Cells from it can penetrate 



tissue; 55 skeleiogenous Q^g notochordal sheath, converliui; it into 



tissue; L b, io\xer process 01 



(skeletogenous tissue.) likrous cartilage, thus enakling it to participate 



in the formation of the verteknu. 



Since the notochord and its envelopes are elastic and give under the 

 strain of the muscles, they are nnsegmenled. Tlie segmentation of the 

 axial skeleton begins with the appearance of lirmer tissue as carti- 

 lage and kone. Then tliere is a separation of successive parts, and with 

 this the gradual formation vi \ertekr;d cohimn and skull. In both 

 tliere is a connected series of modihcations, whether studied onto- 

 genctically or comparatively, from the lower to the higher forms. 



The first parts of the vertebral column to appear are the upper 



I'^lG. 5 1 1 . — Trans\ ersc 

 sceLiou of axial skeleluu oi. 

 Piiromyzoii [irom ^\'le^lers- 

 hcim). C, notochord; t's, 

 notochordal sheath; I-',:, 

 elas'ica externa; F, fatly 

 tissue; .V, spinal cord; i', 

 its meninges; Oh, upper 

 process of skeletogenous 



