HYPERTROPHY, HYPERPLASIA, METAPLASIA, REGENERATION. 105 



formed cells. The sprouts may extend for a long distance into the sur- 

 rounding region, whether this be already organized tissue in which the 

 vessels are increasing in number or unorganized lifeless material like blood 

 clot, which is to be replaced by new living structures. If a similar proc- 

 ess be in progress in neighboring vessels, the sprouts may unite at their 

 extremities, forming a slender solid protoplasmic bridge from vessel to 

 vessel. The sprouts now become thickened and gradually channelled 

 out at the base by pressure of the blood in the vessel from which they 

 spring. The blood enters these lengthening channels, forcing its way 

 along them, forming a lumen as it goes. Simultaneously with this ad- 

 vance of the lumen, new nuclei are formed by division of the old along 

 the sides of the vessel, and the new structure gradually assumes a dis- 

 tinctly cellular and vascular character. At length the channel is com- 

 plete; the new vessels have well-defined eudothelial walls and connec- 

 tive-tissue cells from without, or new connective-tissue cells which have 

 been formed about the nuclei of the protoplasmic sprout, range them- 

 selves outside along the walls. So the new vessel takes its place in the 

 vascular system of the part. Thus, in a short time, many new blood- 

 vessels may form, furnishing nutritive centres about which the organiza- 

 tion of tissue proceeds. 



In the new formation of arteries it is believed that the smooth-muscle 

 tissue is formed by a growth along the developing vessel from a pre- 

 existing artery. 



Regeneration of Cartilage and Bone. New cartilage may form by the 

 proliferation either of connective-tissue or, to a slight extent, of carti- 

 lage cells and the formation about the new cells of the characteristic 

 basement substance. 



The new formation ofjbmie^ under pathological conditions takes place 

 by the development, first, of fibrillar connective tissue or of hyaline car- 

 tilage. These tissues, especially through the influence of periosteal cells, 

 undergo by metaplasia a gradual conversion into characteristic bone 

 tissue. This mode of bone formation is essentially similar to that in em- 

 bryonic development. See Fig. 479, p. 727. 



Regeneration of Fat Tissue. Fat tissue may be farmed by the accu- 

 mulation of fatjlroplets in the cells of various types of connective tissue, 

 particularly in the young or embryonic forms ; but adult connective tissue 

 may change directly into fat tissue. The repair of fat tissue takes place 

 by the formation of young fibrous tissue, whose cells and stroma gradu- 

 ally assume the type of fat tissue. New fat tissue which replaces atro- 

 phied organs or parts of organs, such as kidney, heart, lymph nodes, etc. , 

 is formed in the same way. 



Regeneration of Blood. The formation of leucocytes appears to occur 

 chiefly in the masses of lymphoid tissue which are so widely scattered in 

 the body in the lymph nodes, in the spleen, and in the bone marrow. 

 Both mitotic and v ainitotic_ cejl division are to be observed in the new 

 formation of leucocytes, but the exact relationship between the new cells 

 produced in these" two ways, and their respective destinies, is not yet 



