54 COMPARATIVE ANATOMY. 



off, and, like the manubrium, ossified from a special centre. Thus, 

 in Man, an embryonic stage exists in which the eighth pair of ribs 

 are connected with the xiphoid process. 



BIBLIOGRAPHY. 



GOTTE, A. Bcitrage zur vergl. Morphologic dcs Skclctsystcms dcr ]l'irbdti. 



Arch.f. mikr. Anal. Bd. XIV. 

 HASSE, C., arid BOKX, G. Ucmcrkuij''ii />. d. Morphologic d. liippen. Zool. Anx. 



1879. 

 HOFFMANN*, C. K. Seitrage eur vergl. Anatomic dcr WirbeUhiere. Niederl. .//>//. 



/. Zooloyit, Bd. IV., V. 



LINDSAY, BKATKKT.. Un the Avian Sternum. /'/<. Zool. Soc. 1885, p. 0>1. 

 I'AUKER, W. K. A Mn{><jr<ij>h on the Structure and Development of tlte .s/," 



Girdle and the Sternum. Hay Xoc. 1867. 

 RUGE, G. Untcrsuch. ub. Entivick. am Uruslbeinc d. Menschcn. Morphol. Jahrbuch, 



Vol. VI. 1880. 



IV. THE SKULL. 



Theory of the Segmentation of the Skull. 



In the skull, as in the vertebral column, three stages may 

 be distinguished ontogenetically as well as phylogenetically, 

 viz., a membranous, a cartilaginous, and a bony stage. 

 There is thus an important correspondence between these t\vo 

 parts of the vertebral axis, which is considerably increased by 

 the following facts. 



The notochord always extends for a certain distance into the 

 base of the skull, so that the latter has a similar origin to, and is 

 developed as a direct continuation of, the vertebral axis. Still 

 more important is the fact that a series of mesoblastic somites 

 (protovertebrte) give origin to the greater part of the head as 

 well as to the main dorsal section of the trunk in the embryo, 

 so that both show a metameric mode of origin. Out of these 

 somites, each of which encloses a cavity originating from the 

 ccelome, are formed the muscles of this region as well as the 

 foundation of the proper cranial capsule. As development 

 advances, the original segmented arrangement gradually disappears, 

 and thus the cranium, especially in the lowest Vertebrates, as, for 

 instance, in Cartilaginous Fishes, forms a continuous structure. 



A cartilaginous system of arches, which often later become 

 ossified, arises in serial order on the ventral side of the brain-case : 

 these encircle the anterior part of the alimentary tract like hoops, 

 and are distinguished from the cranial region as the viscera 1 

 skeleton. The latter stands in important relation to gill-breathing, 

 inasmuch as each consecutive pair of arches enclose a passa-e 

 (gill-slit) communicating between the pharynx and the exterior; 

 this is lined by end"derm, and through it the water pas.-es. 

 The foremost visceral arch bounds the aperture of the mouth, and 



