ZOOGENESIS 



these evolutionary trees end in the numerous forms of 

 animal life we know today. 



What was the origin of these basic forms of life, 

 and how did the original representative or representa- 

 tives of each of the several phyla or major groups of 

 animals come into existence? 



All animals have the body composed either of a 

 single cell, or of a multitude of cells all of which are 

 essentially the same in structure. Furthermore, all 

 animals in which the body is composed of numerous 

 cells begin their independent life as a single cell, 

 which divides into two, four, eight, sixteen, and so 

 on, until the full number of cells is reached (figs. 

 110-117; 1x1-1x5, p. 185). 



Since all animals, no matter what they are, begin 

 life as a single cell, it is clear that all animal forms 

 must be interpreted in terms of a primitive single 

 cell. 



A single cell cannot increase in size beyond a certain 

 point without serious interference with the chemical 

 and physical interchanges on which life depends. On 

 reaching the maximum size permitted by the chemical 

 and physical restrictions, the animal cell divides 

 into two; later these two cells each divide into two, 

 becoming four, these four become eight, these eight 

 sixteen, these sixteen thirty-two, and so on in- 

 definitely (figs. 110-117, p. 185). 



In this process of division there are three paths that 

 may be followed (fig. A, p. 2.38). As they divide the 

 cells may separate from each other so that the indi- 

 vidual animals always remain composed of a single 



