When eggs of Ascaris lumbricoides Lin. or A. suilla Duj. containing mature embryos gain entrance to the alimentary canal of the sewer rat, Mus decumanus, or the mouse, M. musculus, they hatch. It is not possible to say at present in what part of the alimentary canal hatching takes place. (The number of animals suitable for experiment at the disposal of the writer is unfortunately so small that he is unable to devote any of them to the working out of the details of the process.) A certain proportion of the larvae thus liberated escape in the faeces where under suitable circumstances they can live for at least three days. It is, however, probable they ultimately succumb and that this is not a true road of development.

The majority of the larvae gain entrance into the body of the host. The exact point of entrance and the time after hatching at which entrance takes place have not been determined. Some animals show signs of illness on the second day after infection. The time elapsing between infection and the entrance of the larvae into the body is therefore probably not more than two days.

Larvae are found in the lungs and liver of the host not later than four days after infection and possibly as early as two days. Sections of the tissues show that they are situated in the air vesicles of the lung and in the blood capillaries of the liver close to the interlobular branches of the portal vein.

Larvae are not found in the liver after the fifth day from infection. They are found in the bronchi about the seventh day and in the trachea on the eighth day.

No larvae are found in any portion of the lung on the ninth day after infection. Dead larvae have been found in the stomach and rectum on the ninth day after the last infection.

The route by which the larvae reach these sites is of course not definitely proved but from anatomical considerations it is hardly possible that it can be other than one of the two following (the diameter of the larva is three times that of a red blood corpuscle of the mouse. The larva could therefore not pass through the lumen of an ordinary capillary vessel):

(1) Boring through the wall of the stomach or intestine the larva enters a mesenteric venule and is carried to the liver. It is here arrested at the entrance to the hepatic capillary plexus and it is for this reason that so many larvae are found in the capillaries close to the interlobular veins. The liver undergoes extreme and acute fatty degeneration so that the larvae are able to penetrate along the capillaries between the degenerated columns of liver cells to the hepatic venules. Thence they pass in the hepatic vein to the heart and by the pulmonary artery to the lung. They are of course at once arrested by the pulmonary capillary field. Embolism of the smaller branches of the pulmonary artery takes place with haemorrhage around these arterioles. The larvae readily work their way along with the effused blood into the air vesicles and thence into the bronchi and trachea.

(2) The larva after hatching in the stomach or duodenum travels up the bile duct and reaches the bile capillaries of the interlobular zone. It here bores its way through the degenerated liver tissues and reaching a hepatic venule continues its course as in the first case.

During the residence in the body of the rat or mouse the larvae grow from a length of 0·22 mm. to 1·4 mm. The proportion length of oesophagus/total length diminishes from 1/2·5 to 1/6·1. The ventral line which is the greatest of the longitudinal lines in the embryo is reduced to the same dimensions as the dorsal and lateral lines. The ventral gland (the rudiment of the excretory system) is developed from a cell of the ventral line, enlarges very greatly, acquires the massive nucleus characteristic of Ascaris larvae (Stewart, 1906; Baylis, 1916) and finally develops its duct from the cells of the ventral line. The intestine, anal canal and anus become pervious. The rudiment of the female gonads appears.