48 GENERAL PART. 



cell membrane. The latter is often without an opening for the 

 entrance of solid bodies ; the entrance of food being entirely effected 

 by endosrnosis. In such cases, e.y., in the Gregarines and parasitic 

 Opalines, the outer body-wall suffices, like the membrane of the cell, 

 for the performance of such vegetative functions as the absorption 

 of food and the removal of the excretory products. The protoplasm 

 (Sarcode) constitutes the body parenchyma, and is the seat of the 

 animal and vegetative vital activities. 



Accordingly there results a definite connection between the 

 functions of the peripheral layer and of the included mass, in which 

 the processes of animal and vegetative life are carried on. This 

 connection pre-supposes a definite relation between the superficial 

 area of the surface and the size of the mass, and this relation changes 

 as growth proceeds. For while the surface increases by squares, the 

 mass increases by cubes ; while the mass increases in three dimensions, 

 the surface only increases in two, and therefore as growth proceeds 

 the relation changes- .to the disadvantage of the latter. In other 

 words, with increase of size the superficial area becomes relatively 

 smaller. Finally it becomes relatively so small that the vegetative 

 processes cannot be carried on, and it is necessary for the mainte- 

 nance of life that for a given energy of life it should be increased 

 by the production of new surfaces. 



This holds not only for the simple unicellular organisms, which 

 resemble cells in their nutritive processes, but also for cells them- 

 selves whose size never exceeds certain fixed limits. Further, as 

 the organism increases in size, not only does it divide into several 

 cells, but these cells arrange themselves in such a way as to give 

 the largest possible extent of surface. The cellular organism accord- 

 ingly acquires not only an outer but also an inner surface on which 

 the cells are arranged in a regular layer. With the appearance of 

 an inner surface, a division of labour is established. The outer layer 

 carries on the animal functions and such vegetative processes as 

 those of respiration and excretion, while the inner (digestive cavity} 

 serves for the reception and digestion of food. 



We thus not only see that increase in size must be accompanied 

 by an increase in the complexity of organization, but also bring out 

 at the same time the essential characteristics of animal organization. 



The numerous cells developed from the original simple organism 

 were at first equivalent to one another, and all endeavoured to take up 

 a peripheral position (colonies of Protozoa Volvox Blastospheere) (fig. 

 40, a, b.) Then, in consequence of the needs of the growing organism, 



