THE CELLULAR CHANGES OF AGE 81 



called the middle-kidney, and which is the only renal 

 organ found in the adult, for the head-kidney disap- 

 pears in these animals long before the adult condition 

 is reached. In the mammal there is yet a third kidney. 

 We have during the embryonic stage of the mammal 

 always a well-developed excretory organ which cor- 

 responds to the middle or permanent kidney of the 

 frog, yet during embryonic life the greater part of this 

 temporary structure is entirely destroyed. It is dis- 

 solved away and vanishes, leaving only a few remnants 

 of comparatively little importance in the adult. The 

 new structure, the permanent kidney which we have, 

 takes its place functionally. Large portions of the 

 tissues which arise in the embryo are destroyed at 

 the time of birth, and take no share in the subsequent 

 development of the child. 1 If we follow out with 

 the microscope the various changes which go on in 

 the developing body we see revealed to us a very 

 large number of cases of death of tissues, followed by 

 their removal. Thus the cartilage which exists in the 

 early stages dies and is dissolved away, and its place 

 is taken by bone. Many of the bony elements of the 

 skeleton in the adult, in the embryo exist merely as 

 cartilage, yet the cartilage is not converted into bone 

 but is destroyed and pari passu its place taken by 

 bone. 2 There is overlying the heart of a child at 



1 Reference is made to the after-birth, which includes the structures known 

 anatomically as the umbilical cord, the amnion (the "caul" of the midwife), the 

 chorion lasve, and the fetal placenta. 



2 The conversion of cartilage into bone was studied by many investigators 

 especially between 1845 and 1870, and was the subject of prolonged and ani- 



6 



