The rapidly accumulating literature on the pituitary gonad relationship and the effects of the gonad stimulating extract from the human urine of pregnancy deals almost entirely with animal experimentation. The entire subject of endocrinology owes its advancement from the speculative phase of the past century to its present level chiefly to the experimentalists. Clinical studies sufficiently controlled to permit careful analysis are relatively few.
The pituitary gonad relationship shows wide species differences even in members of the closely related rodent group— mice, rats, guinea-pigs and rabbits — on which most of the intensive work has been done. When information is considered from the many other species that have been studied— the frog, pigeon, duck, chicken, pig, cow, horse, dog, cat and monkey—wide variations in the mechanisms involved become apparent. This emphasizes the fallacy of predicting reactions in the human being from results obtained by animal experimentation in the pituitary gonad