656 THE SKELETAL SYSTEM 



The notochord never extends cranialward beyond the hypophysis and 

 infundibular downpushing from the diencephalon in any of the vertebrates. 

 This meeting place of the hypophysis, notochord, and infundibulum is a con- 

 stant feature of early vertebrate structure from the cyclostomatous fishes to 

 the mammals. In Amphioxus, however, the notochord projects anteriad be- 

 yond the limits of the "brain" (fig. 249D, E). 



2) Origin of the Mesenchyme of the Early Embryonic Skeleton. The origin 

 of mesenchyme in the early embryo is set forth in Chapter 1 1, page 520. 



c. Importance of the Mesenchymal Packing Tissue of the Early Embryo 



The mass of mesenchymal cells which comes to lie between the embryonic 

 body tubes not only forms the primitive skeletal material of the early embryo 

 but it also serves as a reservoir from which later arise many types of cells 

 and tissues, as indicated in the following diagram; 



endothelial cells of capillaries and other blood vessels 



lipoblasts ►fat cells 



^chondrobiasts (cartilage-forming cells) ^chondrocytes 



and cartilage 



, fibroblasts ►fibrous connective tissue 



.osteoblasts^ ►osteocytes and bony substances, including 



Mesenchymal -:^^'^^ dermal bones and the dermal substances of scales 



cells ^Z;;;;]""-*' macrophages ^-phagocytes 



hemocytoblast (free, wandering, mesenchymal cell) 

 V\\^ erythrocytes 

 y^monocytes 

 \ blood platelets 

 white blood cells 

 'myoblasts (for smooth, cardiac, and skeletal muscle) 



In regard to the skeletal system, it is pertinent to point out the fact that 

 wherever mesenchyme exists, the possibility for connective tissue develop- 

 ment also exists. 



B. Characteristics and Kinds of Connective Tissues 



Connective tissues, other than adipose tissue, are characterized by the pres- 

 ence of intercellular substances which become greater in quantity than the 

 cellular units themselves. In consequence, the various types of connective 

 tissue are classified in terms of the intercellular substance present. Excluding 

 the blood, three main categories of connective tissues are found: 



( 1 ) connective tissue proper, 



(2) cartilage, and 



(3) bone. 



