THE RELATION BETWEEN AMITOSIS AND MITOSIS. 99 



ently only recently separated. From this stage up to the period 

 when the ducts begin to assume definite form the appearance of 

 these regions is much the same. The course of development of 

 this area certainly favors the view that the proliferation is due to 

 some localized stimulus or condition rather than to anything in- 

 herent in the cells themselves. In the central portions of the 

 area the nuclei have divided so rapidly that they are very small ; 

 nearer the periphery they are larger and about the outside of the 

 area are nuclei of the same size as other parenchymal muclei. 

 Divisions likewise decrease in frequency from the center toward 

 the periphery. Fig. 2 shows this difference to some extent. 

 The nuclei near the center of the figure which represents approxi- 

 mately the center of the proliferating area are much smaller than 

 those about the periphery. The two large nuclei in the upper 

 right corner are about the size of typical parenchymal nuclei. 

 Fig. 3 is a smaller area from a somewhat later stage and entirely 

 within the proliferating region. 



The later development of the ducts will be considered more 

 fully in connection with other somatic structures. It is typically 

 a process of continued amitotic division although in some indivi- 

 duals an occasional case of typical mitosis occurs. 



IV. THE FORMATION OF THE OVARY. 



From the region of its first appearance near the nephridial 

 canals (Fig. i) the proliferating area gradually extends somewhat 

 toward the median plane and toward one surface of the proglot- 

 tid known as the ventral surface. So far as can be determined 

 there is no appreciable migration of cells through the parenchyma ; 

 each part seems to be formed in si/u, the stimulus to proliferation 

 continually involving more of the parenchymal nuclei and extend- 

 ing in a more or less definite direction. 



As the inner and ventral end of the proliferating area approaches 

 the inner layer of circular muscles it spreads out into a flat- 

 tened somewhat disc-like area exactly as if it had encountered 

 resistance to its growth in the original direction and so had be- 

 gun to spread out in other directions. Fig. 4 shows a stage 

 soon after the disc-like flattened terminal region has appeared. 

 This disc-like terminal portion indicated by o is the ovary. In 



