HYPOBIOSIS IN PARASITIC WORMS 103 



swallowed and reach the stomach and intestine (for the second 

 time) to settle there. Dming this migration the larva moults and 

 develops. When it has returned to the intestine after migration 

 it is in a more advanced developmental stage than when it 

 hatched from the egg. 



The majority of the larvae, however, pass through the capillary 

 mesh of the lungs and enter the aiterial system. They are then 

 carried to various organs and tissues where they are encapsuled, 

 and eventually die. However, if the host happens to be pregnant, 

 a proportion of the larvae find their way to the placenta, traverse 

 it and concentrate in the lungs of the foetus. In this case an 

 interesting phenomenon occurs: contrary to what happens in the 

 adult host, the larvae do not move from the lung but remain 

 there until the young anim.al is born. Presumably lack of oxygen, 

 the necessary stimulus for the continuation of migration, prevents 

 the larvae from moving. After birth, when the young animal 

 begins to breathe, the larvae leave the lung and reach the 

 stomach and intestine by the normal route. This interruption of 

 the normal course of migration by a more or less prolonged 

 resting sojourn in the lung of the foetus may be regarded as a 

 kind of cryptobiosis. 



HISTOTROPIC PHASE OF NEMATODE LARVAE 



It has been long observed that in the case of some intestinal 

 nematode infestations, the larvae do not develop in the lumen of 

 the intestine of the definitive host, but enter the pits of the 

 gastric or intestinal glands where they are sheltered and complete 

 their last moulting. They then usually return to the intestinal 

 lumen and develop to adults. Kotlan^^ proposed the name 

 'histotropic phase' for this phenomenon. 



Numerous nematodes pass through this developmental phase, 

 e.g. species of the genera Trichocephalus, Ascaridia, Trichonema, 

 Hyostrongylus, Trichostrongylus, etc. In his latest paper on this 

 subject, SommervilleiQ has shown that the larvae of the sheep 

 nematode Ostertagia circumcincta enter the glands in the 



References p. 106 



