Hallez (1885) noted that the deprivation of oxygen prevents the formation of the embryo of Ascaris equorum. Zavadovsky (1916) has shown that in water swarming with aerobic bacteria the eggs of this same parasite are arrested in their development in the two or four cell stage. Martin (1913) has made analogous observations on the eggs of Ascaris vitulorum maintained at 33?C. in water which had been boiled for a long time and introduced into tubes covered with oil. The eggs of this parasite had undergone no division at the end of three and one half months. As parasites become more closely confined, the necessity of free oxygen is apparently decreased. Viviparous nematodes in fact live in cavities or tissues where oxygen is relatively rare, such as intestine and muscles (Trichinella); nodules of the esophagus or stomach (Spiroptera of dogs); muscles and nodular tumors (Onchocerca of man). The embryos develop in the uterus of the female worm and often hatch there. It is possible that they obtain part of the necessary oxygen by breaking down different substances found in their own bodies. The respiration of these worms is poorly understood. Whatever be the mechanism of respiration it is quite evident the necessity of free oxygen decreases more and more and comes to a minimum in nematodes of the most complete adaptation to parasitic life. Manifestly the difficulties in the way of studying experimentally the development and oxygen requirements of the embryonic stages of viviparous nematodes are quite insurmountable. A rather extensive literature, however, is to be found on the helminth eggs that undergo their development in the exterior, including a small amount of work on their temperature and oxygen requirements. The eggs of different species of Ascaris have been the favorite material for such studies on account of their abundance and the ease of procuring them. As stated