18 



BULLETIN 817, U. S. DEPT. OF AGRICULTURE. 



Fig. 1. — Ascaris suum. 

 Egg containing fully 

 developed embryo. 

 Magnified 375 times. 



tarded by the fact that they were in an abnormal host. We have 

 observed full-grown Ascaris in a pig 11 weeks old, so that it is evi- 

 dent that the parasites may attain maturity in pigs within 2^ months 

 after infection. We may therefore conclude that Ascaris^ both in 

 human beings and in pigs, may reach maturity 

 as early as 2|^ months after infection. 



DESCRIPTION OF LARVAL STAGES. 



The newly hatched larvae vary somewhat in 

 size, commonly between 0.2 and 0.3 mm. in length. 

 The specimen figured (figure 2) measured 0.22 

 mm. in length, 0.013 mm, in maximum thickness, 

 esophagus 0.09 mm. in length, distance of excre- 

 tory pore from anterior end of body 0.05 mm., 

 and distance of anus from the tip of the tail 0.01 mm. The, diameter 

 of the body is nearly uniform throughout, head rounded, tail conical. 

 On the anterior aspect of the head is a small, rounded knob, the so- 

 called " tooth " of the Ascaris larva. This knob, according to Stiles 

 (1891), is composed of three parts, corresponding to the lips of the 

 adult Ascaris. In living specimens the outlines of the esophagus 

 except posteriorly can not ordinarily be distinguished and the nerve 

 ring is not apparent. This portion of the body is very clear, and 

 free from color or conspicuous granules. The intestinal cells contain 

 numerous small, yellowish-brown granules. The genital 

 primordium is not evident in living specimens. 



Before hatching, the larva (figure 1) is inclosed in a 

 close-fitting, delicate cuticular sheath (shown in figure 2). 

 When the larvae are artificially expelled from the eggs (by 

 applying pressure) they retain the sheath or in some cases 

 cast it off as they emerge from the eggshell. At the anterior 

 end the sheath is supplied with a crown of 

 minute papillae, apparently 6 in number. 

 Under normal conditions it may be presumed 

 that the larva undergoes its first molt at the 

 time of hatching or shortly afterwards, fol- 

 lowing which at least another molt appar- 

 ently occurs before the larva reaches the stage 

 at which it migrates from the lungs to the 

 intestine. 



The larvae in the lungs reach a length of 

 5 to 10 times that of the newly hatched larvae. 

 The specimen shown (figure 3) is from the lungs of a rabbit 10 days 

 after infection. It measures in length 1.13 mm., in maximum thick- 

 ness 0.065 mm., length of esophagus 0.23 mm., distance of nerve ring 

 from anterior end of body 0.11 mm., distance of excretory pore from 



Fig. 2. — A s c ar i s suum. 

 Newly hatched larva in- 

 closed in cuticular sheath. 

 MagTiited 375 times. 



