. Comparative anatomy. Anatomy, Comparative. 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,
. Comparative anatomy. Anatomy, Comparative. 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 Mes Pre Cart •V . ?'— ^ ^ :^.^^^ i ^^^ :-'' A 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
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