284 



Basic Structure of Vertebrates 



Fibrous connective tissue is omnipresent in the adult vertebrate. 

 It invests, supports, connects, separates, or cushions parts of the body. 

 It penetrates throughout massive organs such as a liver or kidney, 

 binding together their delicate tubules or other constituent parts and 

 serving as a basis for entrance and distribution of blood-vessels. In a 

 modified form, it produces layers or masses of fat. 



Every location where cartilage or bone is destined to develop is 

 occupied by mesenchyme. The deeper parts of the skull, the vertebral 

 column, ribs, sternum, and the skeleton of the paired appendages, are 

 first constructed of cartilage. The entire endoskeleton is permanently 



Pre.Cart. 





,■&> 



Fig. 234. Diagrams illustrating formation of 

 cartilage by mesenchyme. (A.) In fishes, according 

 to Studnicka. (B) In mammals, according to Mall. 

 (Cart.) Cartilage; (Mes.) mesenchyme; (Pre. 

 Cart.) precartilage. (Courtesy, Bremer: "Text- 

 Book of Histology," Philadelphia, The Blakiston 

 Company.) 



cartilaginous in elasmobranchs. Cartilage is a direct product of mesen- 

 chyme. Cells of the mesenchyme become cartilage cells (Fig. 234) and 

 deposit the ground substance or matrix of the cartilage. A connective- 

 tissue membrane, the perichondrium, invests the surface of a carti- 

 lage. In the great majority of vertebrates the primary cartilaginous 

 skeletal structures are, in later development, more or less completely 

 replaced by bone. The process of replacement (Fig. 98) involves the 

 destruction of the greater part of the cartilage. The remnants of the 

 cartilage are in the form of a spongy meshwork whose strands be- 

 come calcified and serve as a framework upon which bone-producing 

 cells, osteoblasts (derived from the perichondrium), build up layers 

 (lamellas) of bone. Occasionally an osteoblast becomes enclosed in a 

 minute space (lacuna) between lamellas and remains as a permanent 

 bone-cell. 



