16 GENERAL CONCEPTIONS, 



Correct notions on this subject are so important that a few illustrations may 

 be mentioned. Let us begin with necrobiosis. There are organs whose existence 

 is limited in time, such as the thymus and foetal kidney. These organs attain 

 their full differentiation, and thereafter most of their elements die off and finally 

 are resorbed, most of the organ disappearing. Another familiar illustration is 

 offered by hairs, which die and are shed. Cell death on a large scale is a com- 

 mon phenomenon of the tissues. It occurs in the cartilage both when the cartilage 

 is permanent and, even more conspicuously, when the cartilage gives way to bone, 

 the disintegration of the cartilage cells preceding the irruption of the bone-forming 

 tissues. It occurs among the gland-cells of the intestine, in the pregnant uterus, 

 and in all the tissues of human decidua reflexa. Degeneration in the stricter 

 sense of the ante-mortem and hypertrophic change of cell structure is also of wide- 

 spread occurrence in the healthy body. Perhaps no instance of this is more 

 familiar than the production of horny tissue in the epidermis or elsewhere. That 

 fatty degeneration takes place normally has long been taught, while mucoid and 

 colloid degeneration are so obviously normal that we commonly think of their 

 pathological occurrence as merely an exaggeration of a normal state. Hypertrophic 

 degeneration is an extremely common pathological process, but it also occurs as a 

 normal process, as, for example, in epidermal cornification, as just mentioned, and 

 very strikingly in the production of giant-cells (myeloplaxes, etc.), and on an astound- 

 ing scale in the uterine tissues during pregnancy in many, perhaps all, mammals. 



4. The Removal of Cells. The sloughing off of cells is one of the most familiar 

 phenomena, since it occurs incessantly over the epidermis and with hairs. Its part 

 in menstruation and its colossal role in the after-birth are known to all, and every 

 practitioner is accustomed to look for shed cells in urinary sediment. Large 

 numbers of cells are lost by the intestinal epithelium. The destruction of blood- 

 corpuscles is incessant, and we might greatly extend the list of these illustrations. 

 Owing to the enormous loss of cells to which the body is subject, there is provision 

 to make good this loss.- This provision is called "regeneration," and has been 

 dealt with in an enormous number of investigations. During embryonic life 

 regeneration plays a comparatively insignificant part, and we shall not have to deal 

 with it further. 



Of the four stages of cytomorphosis, the second, or stage of differentiation, is 

 that which will principally claim our attention. But we cannot fully understand 

 the developmental processes unless we also have constantly in mind the normal 

 degeneration and death of cells, even in the embryo. 



Comparison of Larval and Embryonic Types of Development. 



We have seen in the preceding section that the first cells produced in develop- 

 ment from the ovum are undifferentiated, and are capable of development in many 

 and varied directions. The more they become specialized, the more their possi- 

 bilities of further varied development are decreased. It is thus obvious that the 



