io HISTORY OF THE HUMAN BODY 



The differences between unicellular and multicellular organ- 

 isms in these respects may be graphically expressed in the 

 accompanying diagrams. [Fig. 5.] 



In the first of these (a), which represents the condition in 

 the simpler unicellular animals, a cycle of cell generations 

 begins with a conjugation, a procedure during which a part 

 of the nuclear material of the two conjugating individuals is 

 mutually exchanged, the result seeming to be an increased 

 activity of division for some time. The resulting cell genera- 

 tions are followed in the diagram in the case of but one of 

 the two conjugating individuals, that of the other being sim- 

 ilar. Several generations are indicated, as also the chance 

 mortality of individuals, the result of this last being to keep 

 the total number of individuals in each generation approxi- 

 mately the same in spite of the geometrical ratio in which the 

 individual cells tend to increase. 



The two other diagrams (b) and (c) represent a similar 

 cycle of cell generations in two multicellular organisms, male 

 and female, respectively. In these, the cycle begins with the 

 union of a male and female germ-cell, that is, a permanent 

 conjugation between a micro- and a macro-gamete, forming 

 a fertilized ovum. Because of the cellular differentiation due 

 to a necessary adaptation, the male cell is small and active and 

 equipped with a locomotive organ in the form of a vibratile 

 flagellum, while the female cell is more or less immobile and 

 furnished with a large amount of yolk, the food supply for 

 the embryo during its early development, when it cannot ob- 

 tain its own nourishment. After the conjugation there 

 ensues a series of cell generations, as in the other case, with 

 the essential difference that here they remain in organic con- 

 tinuity with one another and form, not independent indi- 

 viduals, but the component parts of a multicellular organism. 

 The number of such generations is often very great, certainly 

 much greater than here represented, and the cells early begin 

 a differentiation of form and function which leads eventually 

 to the formation of all the tissues necessary to build up the 

 adult body or soma. Among those early cells are the 



