23,1 Schwartz: Life History of the Horse Oxyurid 41 



ficial stimuli that are described elsewhere in this paper, are 

 segmented and develop normally, whereas others are nonseg- 

 mented and deteriorate. Eggs that are liberated under such 

 conditions in large masses due to the eversion of the uterus are 

 largely nonsegmented and do not show any developmental 

 changes in vitro. Eggs that are normally deposited by short- 

 tailed females develop normally. 



Development of the eggs proceeds rapidly on the surface of 

 liquid media and it takes place just as rapidly if the eggs are 

 placed on dry glass slides and not protected in any way from 

 loss of moisture content. Development also proceeds normally 

 in eggs that adhere to the sides of glass beakers at a consider- 

 able distance above the surface of liquid media. 



In view of the gluey investment of the eggs they retain a 

 somewhat moist appearance for several days even though they 

 are kept in a dry place and continue their normal development. 

 The gluey coating probably makes possible the retention of 

 sufficient moisture for development. No differences in the ra- 

 pidity of development of eggs kept in dry and moist media, 

 respectively, have been observed ; therefore, it may be concluded 

 that environmental moisture is not an essential factor in the 

 development of eggs of Oxyuris equi, the necessary moisture 

 being contained in the egg and protected against evaporation by 

 the gluey substance that surrounds it. 



While free access of moisture does not appear a necessary 

 environmental factor in the development of the eggs of Oxyuris 

 equi, free access of oxygen is essential, since development was 

 not observed in eggs that sank to the bottom of liquid-containing 

 vessels. In glass beakers, containing floating eggs on the sur- 

 face of water or physiological salt solution, numerous isolated 

 eggs were usually found at the bottom, and these eggs after 

 many days showed no developmental changes beyond those shown 

 in Plate 1, figs. 2 and 3. Meanwhile the eggs floating on the 

 surface developed rapidly and in four days contained active 

 embryos. That nondeveloping eggs at the bottom of liquid- 

 containing vessels were viable and capable of complete develop- 

 ment was shown by the fact that when these eggs were trans- 

 ferred to glass slides, and the moisture was allowed to evaporate, 

 they continued their development and attained the embryonated 

 stage. Furthermore, when the liquid in glass dishes containing 

 nondeveloping eggs at the bottom was allowed to evaporate, 

 development was resumed as soon as the eggs were exposed to 

 oxygen, and motile embryos were formed. It is evident, there- 



