1114 THE REPRODUCTIVE ORGANS 



a new living entity. The former process or regeneration may be 

 participated in by practically any one of the constituents of its several 

 tissues, while the latter or reproduction, is effected by a special group 

 of cells. In fact, the propagation of the species is so important a 

 function that it is generally mediated by a set of specialized cells con- 

 stituting the organs of reproduction. Thus, the cells of a multicellular 

 organism really arrange themselves into two groups, namely, into those 

 mediating its ordinary processes of life and those concerned with the 

 generation of a new organism. Weissman applies to the former the 

 term of somatic cells, and to the latter, the term of germ-cells. As far 

 as the actual life of the animal is concerned, these reproductive 

 units are of relatively slight importance and are brought into play 

 only when new entities are to be formed. But since even somatic 

 cells are able to reproduce their like, this distinction is not absolute, 

 but merely serves to indicate a physiological division of labor of the 

 cells of the metazoan. 



The life of the organism as a whole is limited and so is that of the 

 numberless constituents of its different tissues. Cells are constantly 

 being destroyed, more so in some tissues than in others, and their places 

 are taken by new units. This implies that even the ordinary tissue- 

 cells must possess the power of reproducing their like. Thus, we have 

 previously noted that the red blood corpuscles disintegrate while they 

 traverse the circulatory system, and are constantly being replaced by 

 new cells derived from the red marrow of the bones. A similar regener- 

 ation takes place in the outermost layer of the skin where the squa- 

 mous epithelium is worn away and is restored by newly formed cells 

 of the deeper Malpighian layer. When exercised, the skeletal muscle 

 acquires new cells, and so does the uterus after its reception of the 

 impregnated ovum. The periosteal cells proliferate when the adjoin- 

 ing bone is broken (callus), giving rise to numberless bone-corpuscles, 

 many of which are again absorbed later on. Under ordinary circum- 

 stances, however, some of the adult tissue-cells are quite unable to 

 reproduce their like, which implies that other cells must step in to 

 consummate this process. Thus, a wound in a muscle is usually closed 

 by a proliferation of its connective tissue elements and not by a mul- 

 tiplication of its muscle cells. This gives rise to the formation of 

 scar-tissue. Furthermore, these processes of regeneration are in- 

 variably retarded after middle life and may in fact be abolished 

 altogether. As an instance of this abolition might be mentioned the 

 abortive proliferation of the cells of the periosteum, causing a per- 

 manent separation of the ends of the fractured bone. 



In general, it. may be said that the more highly organized tissues 

 are regenerated with greater difficulty than those of a more elementary 

 kind. This is especially true of the master tissue of our body, at 

 least insofar as the cell-bodies of the different neurons are concerned, 

 because defects of the central gray matter are always repaired with 



