2o8 THE PHYSIOLOGY OF TWINNING 



I. AXIATE REPRODUCTION 



The characteristic feature of axiate reproduction 

 is that it involves a separation, either complete or 

 incomplete, of a single axiate individual into two or 

 more individuals in such a precise fashion that the line 

 of separation between the two products of division bears 

 a definite relation to at least one of the structural or 

 functional axes of the original individual. There are, 

 according to Child, three types of axiate organization 

 among organisms, which are, in the order of their evo- 

 lutionary origin and their ontogenetic development: (a) 

 the radial axis or that involving a gradient of metabolic 

 activity from the center to the surface; (b) the axis of 

 polarity, involving a gradient of metabolic activity 

 running from anterior to posterior end or from animal 

 to vegetal pole of single cells; (c) the axis of symmetry, 

 involving a double gradient running laterally from either 

 median dorsal or median ventral sides, and giving rise 

 to bilateral symmetry. 



There is a type of axiate reproduction associated 

 with each of these three types of axiate organization. 



A. UNICELLULAR AXIATE REPRODUCTION 



Doubtless the most primitive as well as the most 

 universal type of axiate reproduction is that exhibited 

 in the reproduction of cells. All cells, no matter how 

 little differentiated they may be in other respects, have 

 at least a center-surface organization and it is this axis 

 that determines the sequence of events seen in both 

 mitotic and amitotic cell division whether in Protozoa 

 or in Metazoa. In this type of reproduction the essential 

 feature is that division or doubling of cell structures 



