354 OUTLINES OF CHORDATE DEVELOPMENT 



forming a sclerotomal segment. The sclerotome forms the 

 axial skeleton of the embryo (except the major portion of the 

 skull) and the segments are the rudiments of the vertebrte, 

 which thus alternate with the muscle segments, the arrange- 

 ment of which marks the primary segmentation of the embryo. 

 All details of the formation of the skeletal system lie without 

 the scope of the present chapter, and we shall merely call 

 attention to the fact that the skeleton arises in part from the 

 sclerotomes and in part from the general mesenchyme. The 

 vertebral column is the part derived from the sclerotomes. 

 These cells condense and the cartilaginous rudiments of the 

 vertebra begin to appear during the fifth day around the noto- 

 chord (centra) and nerve cord (neural arches). The sclero- 

 tomes of the head somites form the occipital region of the skull; 

 the remainder of the skull is formed from the mesenchyme 

 around the brain and sense capsules. Cartilage begins to 

 form in the skull during the sixth day. The visceral skeleton 

 forms from the mesenchyme of the visceral arches, cartilage 

 appearing here during the sixth day also. The skeleton of the 

 pectoral and pelvic arches and limbs is formed from the mesen- 

 chyme of these regions, cartilage appearing during the sixth 

 and seventh days. The clavicles, like the derm bones of the 

 skull and anterior visceral arches, ossify directly from mes- 

 enchyme, without being preformed in cartilage. (For simple 

 accounts of the development of the skeleton the student is 

 referred to Marshall, " Vertebrate Embryology/ 7 and Lillie, 

 "Development of the Chick," where full references to the lit- 

 erature will be found.) 



VII. THE URINOGENITAL SYSTEM 



In the chick, as in other vertebrates, the excretory and repro- 

 ductive sytems arise separately and come into relation only 

 secondarily. We may therefore begin with an account of the 

 origin of the excretory system, and through this lead to the 

 development of the reproductive system, and to an account of 

 their association. 



