1. Studies were made on the morphology, with special emphasis on the taxonomic characters, of 220 mature male and female Ascaridia lineata (Schneider) from chickens at Manhattan, Kansas, U.S.A., and Cambridge, England.2. Eggs arising in the anterior ovary pass into the posterior uterus; similarly, eggs from the posterior ovary pass into the anterior uterus.3. In young females eggs may be fertilised at any place in the uteri; in adults, fertilisation occurs near the distal ends of the uteri.4. With the aid of a micromanipulator, a small structure in one pole of the mature egg, previously decribed as an opening, an opercular plug in the shell, or as an internal thickening of the shell, was found to be a solid, conical appendage of the vitelline membrane, free from the shell.5. Fertilised eggs develop to the coiled embryo (infective) stage in 16 days when incubated in water at 30° C.; in 1 mm. of water at 33° C., they become infective in 10 days.6. Water cultures of fertile eggs do not develop when kept at constant temperatures of 0° C., or of 10° C.; at 15° C., development proceeds.7. Constant refrigeration of fertile eggs in water cultures for 1 month at 0° C. so lower the vitality that, on being incubated subsequently at 30° C., they divide slowly and soon die; keeping the eggs at 10° C. for 1 month has no deleterious effect on them when incubated subsequently at 30° C.8. In hatching, the embryo may escape from any part of the egg shell, either in the duodenum of the host, or in water cultures; newly hatched larvae swallowed by the chicken seldom become established; infestation normally result from the ingestion of embryonated eggs.9. The habitat of A. lineata is the duodenum, especially the portion which is a few centimetres posterior to the entrance of the bile ducts; determination of hydrogen-ion concentrations showed that the nematodes live in nearly neutral media (av. 6·7).10. Penetration of the duodenal mucosa by larvae 10–17 days old is frequent in young chickens; occasionally, a larva goes on through the intestinal wall to the liver and lungs, but ordinarily, after the 17th day the young worms withdraw from the mucosa into the lumen of the intestine.11. Young A. lineata grew to maturity in 50 days in chickens parasitised when about a month old; in six 8-day periods following hatching of the larvae, the average daily growth in length per period was about 0·12 mm. in the first period, 0·75 mm. in the second, 1·5 mm. in the third, fourth, and fifth periods, and 3mm. in the sixth period.12. At least three moults occur before the adult from of the nematode is developed. Important morphological changes associated with the moults include: after first moult, presence of pre-anal swelling (males), and of anal prominence in both sexes; after second moult, lips with oral papillae and dentigerous ridges, projecting Iateral folds (alae) present in newly hatched larvae replaced by non-projecting lateral lines, females with vulva and shorter tail proportionately, and males with pre-anal sucker and three pairs of caudal papillae; after third moult, external characters similar to those of the adult A. lineata.
