ENDOSKELETON. 33 



from that of Reptiles, or whether it is to be considered as formed 

 independently, that is, as a new acquisition or " neomorph " 

 (Gaclow). 



Thus it will be seen that the exoskeleton tends 

 gradually to disappear as we ascend in the scale of 

 the Animal Kingdom, while, on the other hand, the 

 endoskeleton becomes of greater and greater importance. 



BIBLIOGRAPHY. 



CREDNER, H. Die Stegocephalen (Labyrinthodonteri) aus dcm Rothlirgcnden drs 

 Plauen'schcn Grundes lei Dresden. Zeitschr. der deutsch. gcolog. Gesellsehaft, 

 1881, 1882, 1883. 



FRITSCH, A. Fauna der Gaskohle und der Kalksteine der Permformation Bohmens. 

 Prag. (not yet completed). 



HERTWIG, 0. Ueber Bau und Entwickclung der Plaeoidschuppen und der Zdhne der 

 Selachicr. Jenaische Zeitschr. Bd. VIII. N.F. I. Ueber das Hautskclct der Fischc 

 (3 Essays). Morphol. Jahrb. Bd. II. 1876 ; Bd. V. 1879 ; Bd. VII. 1881. 



MARSH, 0. C. Numerous papers in the American Journal of Science and Arts. 



WIEDERSHEIM, R. Die Anatomic der Gymnopliionen. Jena, 1879. Z-ur Histologie 

 der Dipnoerschuppcn. Arch. f. mikr. Anatomie, Bd. XVIII. 1880. 



II. ENDOSKELETON. 



I. VERTEBRAL COLUMN. 



An elastic rod, the notochord (chorda dorsalis), lying in the 

 long axis of the embryo, between the neural and visceral tubes (see 

 Fig. 7, B), is to be considered as the foundation of the axial skeleton. 

 Consisting of a meshwork of cells, which are early vacuolated, 

 the outer protoplasmic part of the notochord becomes differentiated 

 into a structureless cuticular sheath (the sheath of the notochord, 

 or elastica limitans inter na), which, however, disappears almost 

 entirely after the notochord has ceased to grow (Fig. 20, Cs). 



Outside this inner sheath, a skeletogenous layer is formed 

 round the notochord from that part of the mesoblast which is 

 distinguished as the mesoblastic somites or protovertebrse. 

 Round the outer periphery of this layer another sheath is formed, 

 the outer sheath of the notochord, or elastica limitans 

 externa (Fig. 20, Ee). 



The skeletogenous layer, consisting of fibrous tissue, now 

 extends dorsally over the spinal cord on each side, and thus gives 

 rise to a continuous membranous tube, which is only broken 

 through at the points of exit of the spinal nerves. No proper 

 segmentation, like that seen in the muscular system, is to 

 be noticed in this membranous stage. The first indication of 

 segmentation is the formation of cartilaginous areas in the mem- 

 brano-fibrous mass of the skeletogenous tissue, in the immediate 

 neighbourhood of the notochord : these show a segmental ar- 

 rangement (formation of metameres), and represent the first 



D 



