Studies on Strongyloides. V. Constitutional differences between a homogonic and a heterogonic line of S. rani.

Strongyloides ratti exhibits, as do most if not all other species of Strongyloides, alternative modes of larval development. On the one hand, the rhabditiform larval progeny of the parasite may metamorphose outside the body of the host directly into infective filariform larvae or, on the other, into sexually differentiated free-living adults. These in turn produce offspring metamorphosing into infective filariform larvae morphologically indistinguishable from those produced directly. These two modes of larval development are known as homogonic or direct, and heterogonic or indirect, respectively. There is essential agreement among many investigators who have studied various species of Strongyloides that the direction of larval development is predetermined, i.e., it is an inherent or constitutional function of the egg from which the larva originates. Nevertheless another point of view still prevails regarding the fundamental factors involved in determining the direction of larval development. Periodically, as well as recently (Beach, 1936), it has been contended that the mode of larval development is not determined by the inherent constitution of the egg but by environmental influences which the young larvae encounter during their period of free-living development. The implications of this latter hypothesis are clear, and insofar as they may apply to Strongyloides ratti, are subject to test. If it is true that elements in the fecal environment influence and thus determine the