SECTION 1 



MORPHOGENESIS 



Revised for the Fifth Edition 

 By C. M. JACKSON, M.S., M.D. 



PROFESSOR OF ANATOMY IN THE UNIVERSITT OP MINNESOTA 



CHANGE is a fundamental characteristic of all living things. The human 

 body during its life cycle accordingly passes through various phases of form 

 and structure. In the earliest embryonic phases of development the 

 changes are very rapid, decreasing in rapidity during the later fcetal stages, but 

 continuing at a diminishing rate throughout infancy, childhood and youth up to 

 the adult. Following the acme of maturity, changes continue which lead 

 gradually to senescence and final death of the body. 



This cycle of change in the body depends upon similar changes in its various component 

 organs, each having its own characteristic life cycle. In a few of the organs this cycle is very 

 short, as in some of the organs of the embryo (e. g., mesonephros). Other organs persist only 

 during childhood (e. g., thymus); while the majority continue, with varying degrees of change, 

 throughout postnatal life. The final death of the body is due to the breakdown of some of the 

 essential organs. 



A further analysis reveals the fact that the characteristic life cycles of the organs depend 

 ultimately upon similar changes in their constituent tissues and ceUs. Every cell has a definite 

 life cycle, an early period characterised by rapid and vigourous changes, later periods of differen- 

 tiation and maturity, followed by stages of degeneration and death. This cycle of cell changes 

 has been designated by Minot as cytomorphosis. 



Growth. — Associated with the process of cell differentiation (cytomorphosis), and even 

 more important as a factor in the morphogenesis of the body, is the process of growth. The 

 developmental changes in form and structure of the body are due largely to the unequal growth 

 of its various parts. Growth, like other changes in the body and its parts, depends ultimately 

 upon the characteristics of the constituent cells. 



Fig. 4. — The Ovum of a New-born Child, with Follicle Cells. (After Mertens.) 



Nucleus 



The cell changes during growth may be grouped under two heads. The first, or growth 

 proper, involves merely the enlargement (hypertrophy) of the individual cells and intercellular 

 products. The second includes the muUi'plication (hyperplasia) of the cells, which is accom- 

 plished by mitotic division. Cell division is necessary in cell growth, for otherwise the cell 

 would soon reach a size where its surface (for nutritive, respiratory and excretory purposes) 

 would be inadequate for its mass. In general, however, cell division is most active in the earlier 

 embryonic periods, during which the cells remain small. Later, cell division diminishes or 

 ceases, and growth is due chiefly to enlargement of the cells already present. It is also during 

 the later period, when the cells have ceased rapid division, that the process of ceU differentiation 

 and tissue formation is most marked. 



The principle of the ratio of surface to mass often appUes to the growing organs as well as to 

 the individual cells. To maintain the necessary ratio, the surface area is increased by the for- 

 mation, through looahsed unequal growth, of projections (e. g., viUi or folds) or invaginations 

 (e. g., glands) from surfaces. Innumerable modifications of this principle occur throughout the 

 process of morphogenesis. 



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