50 



VERTEBRATE REPRODUCTIVE CYCLES 



the eutherian mammals. As regards the lower mammals 

 and other vertebrates almost all the experimental work 

 has involved the injection of pituitary extracts containing 

 both F.S.H. and I.C.S.H. Also in many cases a gonado- 

 tropic hormone derived from the placenta has been used 

 instead of that from the anterior pituitary. This sub- 

 stance is much more easily obtained, being prepared 

 from pregnancy urine, and it has a similar biological 

 action 'to the anterior pituitary hormone. Consequently 

 it is sometimes called the anterior-pituitary-like- 

 hormone, or A.P.L.H. for short. A consideration of its 

 function during pregnancy is out of place here. 



Without going into details, it has been found that 

 both the pituitary and the placental gonadotropic 

 hormones are capable of stimulating gonad maturation 

 in the lower vertebrates. This has been demonstrated in 

 immature and anoestrous individuals of species of 

 marsupials, 1*^ birds,8i» i'^^' i^i reptiles,^" amphib- 

 ians,6i» ®2, 168 and fishes. io6» i^' Thus the general con- 

 clusion emerges that a hormone of the anterior pituitary 

 gland affects reproduction in all vertebrate classes, and 

 since mammalian extracts were effective in these cases 

 it is evident that this hormone is neither species- 

 nor class-specific. However, it is often found that 

 extracts of pituitaries taken from the same species are 

 more potent than those taken from other species. ^^o 



It has also been demonstrated in a variety of vertebrate 

 species, including mammals, that when the gonads are 

 mature the actual release of the eggs from the ovary 

 (ovulation), like that of the sperm from the testis 

 (spermiation), is dependent on the pituitary hormones. 

 The release appears to be due to the combined actions of 

 F.S.H. and I.C.S.H., but the precise manner in which it 

 is brought about remains unknown. Since a similar 

 effect is also induced by the A.P.L.H. present in 

 pregnancy urine, this reaction is often used as a test for 

 pregnancy. Until recently the animal most commonly 

 used was the female clawed toad, which" lays eggs a 



