238 CONNECTIVE TISSUE. 



tissue into bone never occurs ; that between these two kinds of com- 

 pleted tissues the intermediate medullary, or embryonal tissue, stage 

 is invariably present. 



For the study of this -highly interesting question the best subjects are the 

 bones of human embryos, between the fourth and seventh months of intra- 

 uterine life, or the correspondingly developed bones of newly born pups or kit- 

 tens. These animals, at the time of birth, are, as I have already stated, 

 advanced in development as far as human beings in the middle of intra- 

 uterine life. Dogs and cats one year of age correspond in development of 

 this tissue to human beings in the twentieth year of life, and dogs and cats 

 ten to twelve years old are as far advanced in this regard as men of sixty 

 years. 



The phases of development of bone can, of course, be studied with 

 greater ease in dogs than in human beings, and a number of facts obtained 

 from the bones of animals have not as yet been established by investiga- 

 tion of human bones. All specimens must be preserved in, and partly or 

 wholly decalcified by, a one-half per cent, solution of chromic acid; many 

 of the blunders of former histologists, regarding development of bone, re- 

 sulted from the examination of dry specimens. 



DEVELOPMENT OP BONE FROM CARTILAGE. 



In sagittal (antero-posterior) sections of shaft-bones of a 

 human embryo, about five months old, we see that both ends 

 of the future bone are composed of hyaline cartilage, containing 

 many medullary spaces. With low powers of the microscope 

 we recognize that the cartilage corpuscles are closely packed 

 together in the outermost portions of the rounded extremities ; 

 next, the corpuscles are less crowded, but arranged, at regular 

 intervals, in globular heaps, and gradually become arranged in 

 rows, as the cartilaginous head slopes toward the middle portion, 

 the future shaft. It is a mistake to suppose that these changes 

 of shape and situation of the cartilage corpuscles are due to their 

 own activity that, -as Virchow has expressed it, the corpuscles 

 " direct themselves into rows." Before the third month of em- 

 bryonal life we can easily ascertain the fact that, from the earli- 

 est stages of formation of cartilage, the rows are present in the 

 middle portion. It is impossible to understand how cartilage 

 corpuscles, being imbedded in a dense and tough basis-substance, 

 could perform active locomotions without the basis-substance 

 having been-previously liquefied. 



(a) Calcification. At a certain point, nearer the middle, in 

 the younger embryo, the basis- substance is found to be the seat 



