48 VERTEBRATE REPRODUCTIVE CYCLES 



Ligula. For reasons not yet understood, such parasitism 

 is accompanied by a great reduction in the size and 

 numbers of the basophil cells, which in turn is associated 

 with the continuing immaturity of the gonads. Such 

 evidence suggests that the hormone secreted by the 

 basophil cells is that which controls the growth of the 

 reproductive system. 



3. THE HORMONES OF THE ANTERIOR PITUITARY 



A great deal of effort has been devoted to the extraction 

 of these hormones in as pure a state as possible, but un- 

 fortunately they are proteins and the large size of their 

 molecules makes the problem of their analysis extremely 

 difficult. Their structure remains unknown, and although 

 potent extracts have been prepared, it is doubtful 

 whether these even approximate to purity. One of the 

 hormones extracted is that which influences growth, 

 and this will not be mentioned again. The other, which 

 may be a single substance or a complex of related sub- 

 stances, is that which stimulates gonad growth and which 

 is probably derived from the basophil cells. 



It is generally believed that there are at least two 

 secretions of the anterior pituitary which stimulate the 

 gonads, and which are therefore known as gonadotropic 

 hormones. Unfortunately most of the research on these 

 substances has been related to the mammals, and 

 relatively little is known of their actions in the lower 

 vertebrates. The effect of one of these hormones on the 

 mammalian ovary is to cause the rapid growth of the 

 follicles containing the eggs, and thus it is known as the 

 follicle-stimulating-hormone, or F.S.H. for short. In 

 males it stimulates particularly the germinal epithelium 

 to cause the production of spermatozoa. It exerts these 

 effects in mammals which are immature, ^S' ^9 which are 

 in anoestrus,i92 or from which the pituitary has been 

 removed. In pigeons it causes a rapid increase in the 

 size of the eggs in the ovary. 126 Thus the main effect of 



