l66 C. M. CHILD. 



differentiate into the ducts and terminal organs. The develop- 

 ment of the ducts does not appear to be an outgrowth from the 

 region first involved, but seems rather to be due to an extension 

 of certain stimuli or conditions in the parenchyma which bring 

 about proliferation and gradual differentiation of all cells which 

 lie within the region affected. I have found it impossible to 

 reach any other conclusion regarding the development of these 

 ducts. Moreover, in the earliest stage of the ducts I have never 

 observed a single case of mitosis or anything resembling it. On 

 the other hand Figs. 2 and 3 of my earlier paper (Child, 

 'oja) show that amitoses are frequent. In the early stages there 

 is a marked difference in size of the nuclei in the central and 

 peripheral portions of the area in which proliferation is occurring. 

 This is very clearly shown in Fig. 2 (Child, 'oja). Here the 

 nuclei about the periphery of the proliferating region are of the 

 same size as the nuclei elsewhere in the parenchyma, but toward 

 the center their size decreases until they are only a small frac- 

 tion of the size of the parenchymal nuclei in general. Evidently 

 proliferation is much more rapid in the central than in the periph- 

 eral regions of the proliferating area. 



In somewhat later stages the rapidity of division apparently 

 decreases and the nuclei of the central regions gradually increase 

 in size until they are almost or quite as large as those about the 

 periphery. At this time the ducts are visible as bands or cords 

 of very numerous nuclei each surrounded by small areas of cyto- 

 plasm. Between the ducts and other portions of the parenchyma, 

 however, no limiting membrane exists nor do the " cells " appear 

 appreciably different in character from those composing the 

 parenchyma. 



From this stage on the differentiation of the walls of the ducts 

 gradually takes place ; muscle-fibers develop, a lumen appears, 

 and nuclear division becomes less and less frequent. 



In Figs. 1-5 (Plates VIII. and IX.) typical portions of the duct- 

 regions before differentiation has begun are shown. Fig. 1 (PI. 

 VIII.) is taken from the lateral end of the developing ducts at a time 

 when the proliferation has extended to a point midway between 

 the longitudinal nephridial canals and the lateral margin of the 

 proglottid. From right to left the figure includes the whole width 



