124 AVES. 



clitoris or organ of sexual excitement. These are some of the Ducks, 

 as Anas clangula, in which the clitoris is upward of half an inch long, 

 but without any groove. It is of more considerable size, and provi- 

 ded with a demi-groove like the penis, in the Brevipennes, as the 

 Ostrich and Cassowary. 



Several Birds furnish a remarkable peculiarity, and one which is 

 interesting in a developmental point of view, namely, in the coexis- 

 tence along with the left, of the right ovarium either in a rudimen- 

 tary or completely developed condition. In the earlier stages of 

 the existence of the embryo within the ovum, it would appear that 

 the two ovaria and oviducts are in general formed of equal size. 

 Upon the right side, however, these structures soon cease to grow, 

 become absorbed, and disappear almost always before the exclusion 

 of the young bird from the egg. In rare cases, however, as in the 

 adult Duck and Goose, rudiments of this arrest of structure are to 

 be seen, and in some Parrots, Eagles, and Vultures also, there con- 

 stantly occur small rudiments of the right ovarium. The Gos-hawks, 

 as Falco palumbarius and nisus, and the Harriers (Circus) have al- 

 ways two ovaria, provided with mature ova at the period of propa- 

 gation. 



As regards the Male organs of generation, the testes of Birds are 

 always double, and situated like the ovaria behind the lungs against 

 the renal capsules. They are either of an elongated or rounded 

 form, and very small during the winter season when the genera- 

 tive functions are at rest, and in small Birds are then so diminutive 

 that they are with difficulty discovered. They become however 

 remarkably turgid at the breeding season, increasing from 20 to 50 

 times their former size, and are particularly voluminous in some 

 Birds, as the Fowls and Ducks. They are then rarely of equal but 

 mostly unequal size, the left being usually the largest. At this period 

 too of sexual excitement, the contorted seminal vessels are seen very 

 distinctly through their external transparent tunica albuginea, and 

 a very beautiful plexus of vessels is expanded over them. From 

 the seminal vessels proceed the seminal ducts, or vasa deferentia, 

 which pass in the form of serpentine canals upon the anterior surface 

 of the kidneys near to the ureters. Occasionally, as in the Passeres, 

 the seminal ducts form a complex skein of contortions like a ball of 

 thread within the pelvis near to the cloaca, into which they open by 

 a double orifice upon two papilliform projections of the mucous 

 membrane. 



The Spermatozoa or seminal animalcules constituting the moveable 



