CHAPTER XXXVI 

 DEVELOPMENT 



"... I compared the cell-growth, by which Nature builds up a plant or an 

 animal, to the glass-blower's similar mode of beginning, — -always witli a hollow 

 sphere, or vesicle, whatever he is going to make." 



Oliver Wendell Holmes. 



OccuREiNG simultaneously with increase in size, are changes in 

 external form and internal structure — the organism develops. 

 Mainly through the brilliant researches of J. Loeb and his school, 

 some light has been thrown on this seemingly mysterious and 

 apparently inexplicable process. The changes taking place are 

 most readily perceived when the transparent eggs of the echino- 

 derms are used as the material on which to experiment, and 

 consequently, our ideas of the processes involved in mammalian 

 development are largely derived from the study of processes 

 in the lower aquatic animals which may or may not be quite 

 analogous. 



'to' 



Cell Division. 



Cell division is the most general of the specific functions of 

 living protoplasm, and it is the basis underlying the differentiation 

 of the comparatively simple structure of the egg into a more 

 complex organism. The division of a cell is a necessary conse- 

 quence of its increase in volume. The metabolic activity of the 

 cell is a function of its effective surface, i.e. its surface must be 

 of such a size compared with its volume that an adequate supply 

 of oxygen can reach the centre of the cell and that all the by- 

 products of cellidar activity can be eliminated with sufficient 

 rapidity. A freely suspended unicellular animal is spherical. 

 Its surface-volume ratio is 3/1. Doubling the radius increases 

 the surface four times while increasing the volume eight times, 

 i.e. SIV='^, i.e. the effective surface has been halved. The 

 immediate result of decreasing the specific surface to a vcdue below the 

 minimal effective value is to decrease the suppJij of oxygen to the centre 

 of the cell and to cause a heaping up of carbon-dioxide and other 

 products of metabolic activity. This has, at least, two effects — 

 {a) The process of development is retarded (law of balanced 

 reactions), (6) The protoplasm becomes acid. The effect of acid 



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