THE MECHANISM OF LIFE 533 



surface and mass with continued growth, and ought to offer rela- 

 tively the least difficulties to a mechanical explanation. 



With the division of the cell into two independent cells, the 

 relation of surface to mass in the two latter is very different 

 from that existing in the large cell before division. The result is 

 that the metabolic relations will again change, and each cell will 

 assume the same condition which the mother-cell had when it 

 arose by division and began to grow into an independent individual. 

 Thus, from one cell-division to another the same cycle of changes, 

 which is conditioned by the growth of the cell-body and the 

 disturbance in the metabolism caused thereby, repeats itself If 

 these changes are slight, they will not be especially noticeable 

 outwardly, except in an increase in size. Most cells are like this, 

 since they simply grow and, when they have reached a certain 

 size, divide. Where, however, the disturbances in metabolism 

 are considerable, they will be expressed also in a change of the 

 external form of the cell-body, constituting a typical development. 

 A large number of free-living unicellular organisms show this, 

 especially those whose cell-body in division breaks up, not into 

 two halves, but into a larger number of parts, or spores. The 

 difference in size between the spore and the adult infusorian is, 

 indeed, very considerable. Hence the differences in the metabolism 

 must also be very considerable, and a somewhat long development 

 is required before the spore becomes an adult. 



Thus the development of the cell, the periodic return of one and 

 the same cycle of form-changes from one cell-division to another, 

 from one spore-formation to another, is seen to be a simple 

 expression of changes in the cell-metabolism that are caused by 

 growth. During growth with the close correlation of all 

 parts of the cell with one another and the constituents of the 

 medium, innumerable other factors, both chemical and physical, 

 must appear, and these must combine with one another to assist 

 and promote the form-changes. But as the fundamental cause of 

 all these changes no other factor than growth need be assumed. 

 It alone suffices to explain their periodic character. 



b. Developmental Mechanics 



A question that has reference to the development of the multi- 

 cellular organism from the ovum by continued division has recently 

 become the focus of active discussion. It is this: — How does the 

 division of a cell into two unequal parts, a circumstance that forms 

 the fundamental condition of the development of every differen- 

 tiated cell-community, come about ? This question, which is 

 fundamentally important in an understanding of the development 

 of all higher organisms, is answered in two very different ways. 

 The view of one class of investigators follows the theory of His 



