158 TROPICAL MEDICINE AND HYGIENE 



Necator americanus was first described in America, but 

 probably is the indigenous species in Tropical Africa. 

 It is the worm found in the pigmies from the Central 

 Forest, in Uganda and on the West Coast. In America 

 it is common, and may have been introduced by the 

 Negro slaves. Both worms are found in many places in 

 Tropical America, as both Indian and Negro labour has 

 been imported, and in the Malay States, Ceylon, and 

 Assam. More information is required as to the species 

 found in the aboriginal races in the Pacific Islands and 

 in South America. 



Life-history of the Worms. The eggs are, when passed 

 with the faeces, in the early stages of segmentation, and 

 under favourable conditions rapidly develop, so that they 

 frequently contain embryos in twenty-four to forty-eight 

 hours. The main conditions required are moisture and 

 heat. Sufficient moisture is always contained in the 

 faeces, the temperature is therefore the main factor to be 

 considered. 



The optimum temperature is a uniform one of 75 F. 

 Below 75 F. development is delayed, and progress is 

 slow and incomplete. Near the freezing point many eggs 

 die, but some survive and will hatch out when exposed to 

 suitable conditions. 



The embryos when hatched out are actively motile 

 rhabditiform embryos. They live in faeces, decomposing 

 or otherwise, for two or three days and rapidly increase 

 in size. If kept in undiluted faeces they do not continue 

 to develop and soon die. In moist earth they live for 

 prolonged periods, months or even years, and continue to 

 develop up to a definite stage. In their final stage of 

 development in this, the free-living, form they are motion- 

 less and enclosed in a thick sheath formed by the dis- 

 carded cuticle. In this condition they will live for months 

 without undergoing further development. 



Various observers have described the development of 

 these embryos into free-living sexual forms, but all care- 

 fully conducted experiments where sterilized earth is used 



