1092 



SUBMAMMALIAN VERTEBRATES 



reviewed by Hutt (1949) ; growth of larger 

 spurs in capons than in roosters indicates 

 that androgen inhibits spur growth. Unilat- 

 eral ovariectomy of the hen leads to spur 

 development, suggesting that estrogen sup- 

 presses spur development to a larger extent 

 than does androgen. Transplantation ex- 

 periments have indicated that the male 

 spur has a greater potential for growth than 

 the female spur. According to Hutt (1949), 

 this is because of greater natural and arti- 

 ficial selection for this character in the male. 

 The role of androgens in regulation of be- 

 havior and its importance in the reproduc- 

 tive cycle is discussed in other chapters. 



3. Seminiferous Tubules 



The seminiferous tubules, in wliich sper- 

 matogenesis takes place, may reach a length 



Fig. 18.1. Spermiogenesi.s in the duck according 

 to Clermont (1958). The dark bkck dot in No. 2 

 represents the acrosome. This acrosomal material 

 attaches to the nucleus which is represented in 

 black and dark grey. (From Y. Clermont, Arch. 

 Anat. micrcscop. et Morphol. e.xper., 47, 47-66, 

 1958.) 



of as much as 250 meters in an adult rooster 

 (Kumaran and Turner, 1949a). As might 

 be expected, the onset of spermatogenesis 

 in domestic fowl depends on the breed and 

 strain. The seminiferous tubules are lined 

 by the germinal epithelium in which occa- 

 sional Sertoli cells are found. Whether or 

 not the Sertoli cells have an endocrine func- 

 tion in birds has not been established. The 

 evidence concerning estrogen production in 

 man (reviewed by Leach, Maddock, Toku- 

 yama, Paulsen and Nelson, 1956) seems 

 to indicate that they are not the source of 

 estrogen. As will be discussed later, the 

 Sertoli cells probably play an important 

 role in the release of sperm into the lumen 

 of the seminiferous tubules. Other functions 

 of the Sertoli cells are not well established, 

 but Ortavant (1959) lists as possible func- 

 tions the protection of certain germ cells 

 and aid in the maturation of spermatids. 



Spermatogenesis has been studied in de- 

 tail for the Pekin duck testis by Clermont 

 <1958). The following account has been 

 taken from his publication. The spermato- 

 genic process can be divided into 3 periods: 

 (1) the period of spermatogonial multipli- 

 cation, during which new spermatogonia 

 are formed continuously and during which 

 spermatocytes are formed; (2) the period 

 of spermatocyte division which results in 

 spermatid formation; (3) the period of 

 spermiogenesis, i.e., the period during which 

 spermatids are transformed to spermatozoa. 



The transformations occurring during 

 spermiogenesis are best illustrated in Figure 

 18.1. The number of each figure corresponds 

 to the different stages which Clermont 

 (1958) distinguishes. Figure 18.2 illustrates 

 the different cell associations formed during 

 the spermatogenic cycle; these findings are 

 summarized in Table 18.1. 



By the use of colchicine injections to stop 

 mitotic division, it was possible to count 

 the number of mitoses in each stage of the 

 cycle listed above. Clermont (1958) con- 

 cluded that the scheme illustrated in Figure 

 18.3 was the most probable one. Lake 

 (1956) made the very interesting observa- 

 tion that in fowl testes two secondary sper- 

 matocytes may remain together to form a 

 lunucleate cell. This cell becomes a four 

 nuclear cell after the second meiotic divi- 



