148 Harry Lewis Wieman. 



of irregular cell activity among the future nurse cells that lend 

 additional support to the idea. 



Do the egg cells cease to multiply because they have passed 

 through a fixed number of cell generations; or does the cause lie 

 in the action of some substance produced in the egg tube that pre- 

 vents further cell division, and turns the direction of cell activity 

 into different channels, the result of which is growth? Much evi- 

 dence favors the latter view, for we are learning more and more 

 that development and differentiation are largely a matter of cor- 

 relation of mechanical and chemical forces. Thus Spemann 

 ('01), Lewis ('04), Le Cron ('07), and others have shown that the 

 development of the lens of the eye in certain amphibia depends 

 upon a stimulus set up in the ectoderm in the region where the 

 outgrowing optic vesicle touches it. In what is known as 

 ''hormone action" we have an example of the secretion of one 

 organ reacting upon another organ in such a manner as to cause 

 secretion in the latter, or to affect its metabolism in other ways. 

 The phenomena accompanying the oncoming of puberty and the 

 results of castration are too well known to need mention. 



Such facts indicate that the interaction, whether by mechani- 

 cal contact or through the production of chemical combinations 

 of tissues or their secretions upon each other is an important 

 factor in developmental processes. Therefore, I believe this semi- 

 liquid condition occurring in the terminal chamber at a definite 

 time in the history of the organ, is to be regarded as a physical 

 and chemical reaction in which the epithelial cells of the egg 

 chamber and those of the ovariole stalk are involved, and that it 

 is an important, if not the causative, factor in the differentiation 

 of the primordial germ cells into egg cells and nurse cells. 



From this it follows that the epithelial cells, which come chiefly 

 from the terminal thread, are very intimately connected with 

 the development of the germ cells, but I do not believe they take 

 any part in forming the follicles of the egg. These are formed 

 from the columnar cells of the tube stalk. In the first place, the 

 epithelial cells are never found below the lower limit of the diffuse 

 area, while the follicles are always formed below this level (Fig. 

 11). In the second place, the folhcle cells (Figs. 11, 12) bear an 



