32 GENERAL CONCEPTIONS. 



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

 attain their full differentiation, their elements during the next stage die off and 

 finally are resorbed, most of the organ disappearing. Another familiar illustra- 

 tion is offered by the notochord, which in mammals totally disappears. Cell 

 death on a large scale is a common 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 strik- 

 ingly in the production of giant-cells (myeloplaxes, etc.), and on an astounding 

 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 fam- 

 iliar 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 illus- 

 trations. 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 em- 

 bryonic 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 under- 

 stand 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 devel- 

 opment from the ovum are undifferentiated, and are capable of development in 



