58 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



or less replaced during development by bone, secreted by bone-forming cells. The skeleton 

 is then said to be ossified. Bone produced in this manner by replacement of pre-existing 

 cartilage is known as cartilage bone. 



Investigation of the internal skeleton of vertebrates shows that not all of their bones 

 arise in this manner, but some of them develop directly from the mesenchyme without pass- 

 ing through a cartilage stage. Such bones are called dermal, membrane, or investing bones. 

 They are really derived from the dermis of the skin and are therefore dermal plates homologous 

 to ganoid scales and to the plates of the turtle's armor. They are consequently parts of the 

 exoskeleton. They have sunk inward from their original position in the skin and have 

 attached themselves to the endoskeleton with which they are now so closely associated that 

 they must be considered a part of it. 



The student should particularly understand that cartilage and membrane bones look 

 exactly alike; they have the same histological structure and chemical composition; they are 

 both completely formed bone; it is not possible to distinguish them by examining them. 

 It is only their manner of origin that is different. In order to determine which bones of the 



P h g 



FIG. 15. Diagrams to show the skeleton-forming septa in A t the tail region, and B, the trunk region 

 of a vertebrate, a, skin; b, neural tube; c, notochord; d, blood vessel; e, dorsal skeletogenous septum; 

 /, ventral skeletogenous septum; g, horizontal skeletogenous septum; h, myoseptum; i, epaxial part of 

 the myotome; j, hypaxial part of the myotome; k, coelom; /, intestine; m-p, cartilages from which 

 the vertebrae are formed m, basidorsal; n, interventral; o, basiventral; p, interdorsal q, inter- 

 muscular rib; r, subperitoneal rib. In B note positions of the vertebral cartilages and ribs with respect 

 to the skeletogenous septa. (A after Kingsley's Comparative Anatomy of Vertebrates, copyright by 

 P. Blakiston's Son and Company; B from Goodrich in Part IX of Lankester's Treatise on Zoology, 

 courtesy of the Macmillan Company.) 



endoskeleton are cartilage bones and which are membrane bones it is necessary to study their 

 embryonic development. This has been done for the majority of the bones of the skeleton. 

 In order to trace the homology of the parts of the skeleton in different vertebrates it is abso- 

 lutely essential to know which bones are cartilage bones and which are dermal. This informa- 

 tion is given in the following pages. For the present we may state that dermal bones occur 

 in connection with the skull, jaws, and pectoral girdle (these parts also contain cartilage bones, 

 of course). All other parts of the endoskeleton are composed wholly of cartilage bone. 



B. THE EMBRYONIC ORIGIN OF THE VERTEBRAE AND RIBS 



i. The development of the sclerotome. The axis of the vertebrate skeleton is the vertebral 

 column or backbone, which is composed of a longitudinal series of bones, each of which is called 

 a vertebra. The vertebrae arise from the sclerotomes. We have already learned that a 

 sclerotome is a mass of mesenchyme originating from the medial wall of each epimere (see 



