<I!I21> piishfd till' i.ott'niiinatioii ;i:ifK still r'lirtliii', l» ilu- dc 

 velopiiift OKRS. Siiiidttrouinl (lil'JO), wlio ro|ioitiMl tlio nndliiR 

 of sperins in frmali' worms jinil romUiili'd that the worms 

 wore liermapliroditic, sii(;K>'sti'd that tlio diri'ition of drvrlop 

 mont is dotormiiiod Ivv tlio cliioinosoiiial constitiilioii of tlio 

 ORRS siibsi>(iU(Mit to firtilizatioii. Kaust (l!i:!3), liaviiiK found 

 what he inti'rprotod as parasitic niaU's. suntri'stcd that fntilizi'il 

 *'BBS givo risi" to hotoioKoiiic and iitifi'rtili/.ed orks to lionio 

 gonic progeny. Snlisecpiontly Beacli (, 1!I,'!.">1 ".).'!(>'), working in 

 Faust's laboratory, showed conelusively that the course of 

 development can l>e iiitluenccd by nutritional conditions; as 

 these become less favorable more and more of the rhabditi 

 form larvae undergo direct development to filariform larvae 

 instead of becominu males and females. The evidence indi 

 catcd that the potential females are intluenced in this way 

 more readily than the potential males. 



Meanwhile Graham (ISKUi l'.)3il) started two pure lines of .S. 

 rata of rats from original single larva infections of the homo 

 gonic and heterogonic type, respectively, and found marked 

 inherent dilTerenees between them. In each line over 85 per- 

 cent of the total progeny were of its own type, with an ex- 

 treme difference in the number of males produced. Graham 

 also observed that there was .-i falling off in heterogonic larvae 

 in winter as comjiared with summer, brought about b.v climatic 

 effects on the host, not on the develojiing larvai'. Tlie conclu 

 sion seems warranted, therefore, that the course of develop 

 ment is dependent upon nutrition or other cnvirontuental con 

 ditious and not upon genetic constitution, but that there are 

 genetic differences in the e.\tent to which different strains 

 are influenced towards homogony by a given degree of unfavor 

 ableness in the environment. 



The reproductive status of the parasitic females was brought 

 into question by Sandground (lilifi^ ; prior to that time it 

 had been generally accejited that they were partheuogenetic, 

 although Leuckart ai>parently suspected that they were hernia 

 phroditie, by analogy with the condition in the parasitic genera- 

 tion of Rhabdia.i. Satulground believed them to be protandrous 

 hermaphrodites; he described what he interpreted as sperms 

 and observed what seemed to be fertilization in specimens of 

 S. ratti. Faust (1933), having found male worms in the lungs, 

 concluded that the sperms observed by Sandground luobably 

 were the result of copulation. He considered the worms to 

 be bisexual early in life, later becoming parthenogenetic. Chit- 

 wood and Graham (1940) concluded that -S. ratti was parthe- 

 nogenetic since they were unable to find sperms and also un- 

 able to find fertilization membranes. The weight of evidence 

 is therefore in favor of parthenogenesis. 



The occurrence of parasitic males described by Kreis (1932) 

 and Faust (1933) has not been confirmed by others. In an 

 unpublished observation, one of us (J. E. A.) has noted adult 

 rhabditiform males in the fresh feces from a ease of human 

 strongyloidiasis but it was unknown whether these were para- 

 sitic males or males developing from eggs of parasitic females. 

 The fact that the supposed parasitic males of Kreis and of 

 Faust were rhabditiform and practically identical with free- 

 living males is sufficient cause for doubt that they are really 

 males of the parasitic generation, for in the one other mem- 

 ber of the Strongyloididae in which males have been found — 

 Parastronffyloidcs uinchcsi, Morgan 1928 — the parasitic males 

 are filariform like the females. We suggest that, since Faust 

 not only observed eggs and rhabditiform larvae, but also 

 filariform larvae which he interpreted as the progeny of 

 the parasitic worms, in the lungs and bronchioles of infected 

 hosts, the males observed were free living males produced 

 precociously in the lungs. The observations of Beach (1. c.) 

 that males will develop more readily than females under subop- 

 timal conditions would account for the failure to find free- 

 living females. Graham's work with single-larva infections 

 has shown clearly that males are at least unnecessary in S. ratti, 

 though no conclusions can be drawn from this, for it is, of 

 course, possible that there might be differences between spe- 

 cies in this respect. For the present the occurrence of rhab- 

 ditiform parasitic males in members of the genus Strongy- 

 loides must certainly be considered sub jtidice. 



Fig. 180. DEVELOPMENT OF STRONGTLOIDKS 



A-G — StTontryloiileg stercorals (A — Parasitic female; B — Free living 

 male; C — Free living female; D — Filariform larva (human strain A), 

 from lung tissue of experimental dog three days after skin inoculation; 

 E — Post-filariform larva from same: F — I'readolescent female from lung 

 of experimental dog 11 days after skin inoculation, note developing 

 genital primordium; G — Adult male from lung tissue of experimental 

 dog 57 days after skin inoculation). H-M — Strongyloitle/t sp. from dog. 

 free living generation (H — First stage larva; I — Infective larva: J — 

 Tail of infective larva; K — Head, adult female; L — Adult female; M — 

 Tail, male). A-C. after Faust. E. C, Human Helminthology. 1939. 

 D-G. after Faust. E. (',. 1933. Am. J. Hyg. v. 18 (1). Remainder 

 original drawings by M. B. C. 



271 



