376 



PHYSIOLOGY OF GONADS 



III) . No function can be ascribed to tlie se- 

 cretion. 



3. Seminal Vesicles 



The seminal vesicles are paired, usually 

 elongated glands which may appear rela- 

 tively simple externally (Figs. 6.2 and 6.3) 

 but are subdivided internally by compli- 

 cated villous projections. The name refers 

 to an old misconception that they are sperm 

 reservoirs. The seminal vesicles are rela- 

 tively enormous and distended with secre- 

 tion in some mammals, for example, the rat, 

 guinea pig, and hamster; they are large in 

 others such as the boar, and in still others, 

 as in man, they are small and compact. 



Seminal vesicles are absent from the mon- 

 otremes, marsupials, carnivores, and ceta- 

 ceans that have been studied, and from some 

 insectivores, chiropterans, primates, and 

 lagomorphs. Variability exists among the 

 edentates ; the sloths and the armadillo have 

 seminal vesicles which are well developed 

 in the two-toed sloth and armadillo, but 

 are very small and rudimentary in the 

 three-toed sloth. Among the lagomorphs, the 

 seminal vesicle of the domestic male rabbit 

 is a large unpaired gland whereas the semi- 

 nal vesicles in the adult cottontail rabbit 

 are vestigial or absent although they de- 

 velop for a time in the fetus (Elschlepp, 

 1952). 



4- Ampullary Glands 



These organs are glandular enlargements 

 arising from the ampullae of the ducti 

 cleferentes or the posterior region of the 

 ductus if a distinct ampullary enlargement 

 is not present. They may be only slight 

 glandular enlargements of the wall, or dis- 

 crete glands which nearly encircle the duc- 

 tus deferens as in rats, some mice, and ham- 

 sters (Fig. 6.2). They are vestigial in 

 certain pure line strains of mice (Horning, 

 1947) and lacking in guinea pigs (Fig. 6.3). 

 In some bats, they attain very large size. In 

 general, they are absent from many mam- 

 malian orders and variable in others (Table 

 6.1). 



D. EVOLUTIONARY HISTORY OF ACCESSORY 

 REPRODUCTIVE GLANDS OF MAMMALS 



The well developed male accessory glands 

 which characterize the niamnialiaii class as 



a whole, and form such a conspicuous part 

 of the reproductive tract in most mammals, 

 are not found in nonmammalian vertebrates. 

 These glands appear as anatomically dis- 

 tinct organs in the primitive prototherian 

 mammals, the monotremes, which are defi- 

 nitely mammalian but which also retain 

 certain anatomic characteristics of their 

 reptilian ancestors and still lay shelled eggs. 

 However, it has been suggested that in the 

 evolution of the three groups of living mam- 

 mals from mammal-like reptiles, the line of 

 descent of monotremes is entirely separate 

 from that of marsupials and placentals. 

 Furthermore, the last two groups are prob- 

 ably parallel branches of the mannnalian 

 stock. 



The accessory glands of modern mam- 

 mals represent, then, the parallel evolution 

 of discrete glands that probably began their 

 development very early in the evolutionary 

 history of mammals. In gross structure, size, 

 and internal complexity they are unique 

 accessory organs among vertebrates. Mod- 

 ern reptiles have no such glands; the semi- 

 nal plasma is composed mainly of secretions 

 from the epididymis and the renal tubules 

 of the sexual segment of the long, lobulated 

 kidney. Both these regions become highly 

 secretory during the breeding season (see 

 chapter by Forbes) . Parenthetically, the se- 

 men of birds (a later offshoot from the rep- 

 tilian line than mammals) contains only a 

 small amount of seminal plasma (Mann, 

 1954a) which is secreted in the cock almost 

 entirely by the seminiferous tubules and 

 vasa efferentia (Lake, 1957). In modern 

 mammals, the epididymal epithelium is 

 still an important accessory secretory area 

 (see chapter by Bishop) , but the bulk of the 

 seminal plasma comes from glandular elab- 

 orations of quite different regions, the uro- 

 genital sinus (a derivative of the primitive 

 cloaca) and the posterior part of the Wolf- 

 fian ducts, the ducti deferentes. 



Modern monotremes are specialized forms 

 but in certain characteristics they are primi- 

 tive. They show almost diagramatically 

 some of the first steps in the evolution of 

 accessory glands. The bulbo-urethrals are 

 already well developed but the concentra- 

 tion of complicated urethral glands at the 

 neck of the bladder in tlie duckbill platypus 

 almost certainlv illustrates the derivation 



