272 GEO. W. TANNREUTHER. 



dermal and endodermal cells were similar to those in hydra dur- 

 ing the formation of the sexual organs. This would indicate 

 that lack of food and production of sexual organs give similar 

 effects. Sexual organs never appeared on well-fed hydras, except 

 those that had passed through a vigorous process of budding for 

 a definite time. This indicates very forcibly that food is not a 

 direct controlling factor in the production of sexual organs, but 

 instead favors vigorous budding, which in turn gives rise to con- 

 ditions that cause the appearance of these organs in the animal's 

 life history. The question immediately arises, what are these 

 conditions ? Is it some inherent factor within the interstitial cell 

 that has to do with the appearance of the sexual organs irre- 

 spective of food, temperature, etc.; or is it perhaps the inability 

 of the cells in general to assimilate the food present after an active 

 process of budding which leads to the differentiation of sexual 

 organs in some definite region so that a new cycle may be 

 started ? The latter view seems the more plausible, as the 

 ectodermal cells do pass through a marked change in those 

 hydras that have been budding actively for some time. The cells 

 become less granular, numerous vacuoles appear, the nuclei stain 

 less intensely and very seldom divide. These conditions, how- 

 ever, vary in different parts of the same hydra. Moreover, as 

 the ectodermal cells show this somewhat degenerate or exhausted 

 condition in the parent hydra that is actively feeding and bud- 

 ding, it suggests very forcibly, as stated above, the inability of 

 the ectodermal cells to assimilate the food present. 



Summary. 



The interstitial cells which give rise to the ovary, after increas- 

 ing in volume become differentiated into two distinct regions : a 

 central region which contributes directly to the formation and 

 growth of the ovum, and a peripheral region whose nuclei later 

 enter the egg and become changed into yolk. 



Occasionally two nuclei persist in the central region and give 

 rise to two distinct ova. Each ovum has its individual membrane 

 and is entirely independent of the other. The ova are forced 

 through the small opening of the ectoderm to the exterior by the 

 contraction or pulling back of the ectoderm. The polar bodies 



