46 VERTEBRATE REPRODUCTIVE CYCLES 



At the end of the chapter the question of the external 

 environment is discussed again, and an attempt is made 

 to indicate the manner in which it exerts its effect on the 

 internal physiological mechanism. 



2. THE PITUITARY GLAND 



The organ which apparently holds the keys to the re- 

 productive processes is the pituitary gland, or hypophysis, 

 and although the details of its anatomy may vary widely, 

 its basic structure and function are much the same in all 

 vertebrates. In the adult it is seen as a glandular mass 

 attached by a stalk to the base of the brain, but a study 

 of its development shows that it is a composite organ 

 formed partly from an invagination of the ectoderm of 

 the buccal cavity (the so-called Rathke's pit) and partly 

 from an evagination of the floor of the forebrain. In the 

 cyclostomes these two components may remain separate 

 throughout life, but in the higher types they are in- 

 timately fused together. 



In its adult form the organ consists of two main lobes, 

 the anterior and the posterior. The anterior lobe is 

 derived from the anterior side of the buccal invagination 

 while the posterior lobe is formed partly from the poster- 

 ior side of the buccal invagination (which is also called 

 the intermediate lobe) and partly from the nervous tissue 

 which retains its connexion with the base of the brain 

 (and which is also called the neural lobe). The cleft that 

 separates the simple anterior lobe from the composite 

 posterior lobe is all that remains of the cavity of the 

 buccal invagination, the cavity of Rathke's pit. 



The various parts of the pituitary gland secrete a 

 number of hormones, but for present purposes only 

 those of the anterior lobe are important. The cells which 

 compose this lobe are of three kinds, and they tend to be 

 mixed indiscriminately together. According to their 

 staining reactions they are known as acidophils (com- 

 bining with acid stains), basophils (combining with 



