REPRODUCTION IO3 



parts of the body. Its microscopically delicate ramifications penetrate 

 throughout such massive structures as a liver or kidney, binding together 

 the exceedingly tenuous tubules of those organs and serving as a basis 

 for entrance and distribution of blood vessels. In a modified form it 

 becomes bulky masses of subcutaneous fat. Whale blubber is pre- 

 sumably a derivative of mesenchyme. 



Skeleton. 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 



Cart 



r^ Q 



Fig. 76. — 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. (From Bremer, Text-book of Histology.) 



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

 chyme. Cells of the mesenchyme become cartilage cells (Fig. 76) and 

 deposit the ground substance or matrix of the cartilage. 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 involves the destruction of the greater part of the 

 cartilage. The remnants of the cartilage are in form of a spongy mesh- 

 work whose strands become calcified and serve as a framework upon which 

 bone is deposited (Fig. 77). The bone-producing cells, osteoblasts, 

 seem to be derived mainly, if not entirely, from the perichondrium, the 

 membrane which invests the surface of any cartilage. In the develop- 

 ment of one of the long bones of an appendage, the perichondrium 

 produces a shell of bone upon the external surface of the cartilage (peri- 

 chondral bone) while extensions of the perichondrium burrow into the 

 degenerating cartilage and provide the osteoblasts which build up endo- 

 chondral bone on the calcified remnants of the cartilage. The process of 

 ossification involves at first only the middle region or shaft of the devel- 

 oping long bone. The terminal regions (epiphyses) remain for some time 

 cartilaginous and do not become wholly ossified until growth is completed. 



