906 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



are also in the strict sense of the word oviparous animals, only in them 

 the egg does not leave the body of the female until completely developed. 



In mammals the fecundated egg passes through the Fallopian tube 

 and becomes arrested in the uterus and is there fixed, and there under- 

 goes what may be termed an internal incubation, while at the same time 

 vascular connections between the e'gg and the body of the mother are 

 established. In birds, likewise, the fecundated egg passes through the 

 oviduct and there becomes coated with an albuminous la3 r er, around 

 which is finally deposited a layer of calcareous matter which hardens 

 before the egg is extruded. It, therefore, contains within itself the 

 materials necessary for the development of the embryo, and is, as a con- 

 sequence, more voluminous than that of the mammal. The eggs of birds 

 are finally developed by external incubation. 



Birds, like reptiles, are, as a rule, possessed of no external male 

 organs of copulation. The testicles are placed near the kidneys, and the 

 spermatic canals open at the inferior extremity of the digestive tract at 

 the cloaca, and it is by the application of the anus of the male to the 

 cloaca of the female that fecundation is accomplished. In certain birds, 

 as the ostrich, duck, and goose, a rudimentary penis is, nevertheless, 

 present. 



The fundamental part of the egg, or the yelk, is formed in the ovary 

 of the female. When the yelk has attained its full development the 

 ovarian capsule breaks and the yelk, inclosed by the vitelline membrane, 

 passes into the oviduct. There it meets with the seminal fluid and becomes 

 enveloped by a layer of albuminous matter, while the yelk undergoes a 

 rotatory movement and the chalazee, or albuminous ligaments, found 

 in the white of the egg are formed. About six hours after the exit from 

 the ovary, and when the egg has reached the lower third of the oviduct, 

 the albuminous layer, or the white of the egg, becomes enveloped by a 

 membrane, at first transparent, and which finally doubles itself into two 

 folds. The fold adhering to the albumen remains in the state of a mem- 

 brane, while calcareous matters are deposited in the more external 

 membrane so as to form the egg-shell. The formation of the shell is 

 much slower than that of the albumen, and it is only after about twenty- 

 four hours that the complete egg is expelled from the inferior part of the 

 oviduct into the cloaca and thence to the exterior, the small point of the 

 egg being first extruded. 



If examined while still within the oviduct, or immediately after its 

 extrusion, it may be readily determined that characteristic changes have 

 occurred within the germinal vesicle, and these progress until the embryo 

 is completely developed, its life being sustained during the period of 

 incubation by the albuminous matters stored up in the egg, while respi- 

 ration takes place through the pores of the shell membrane. In the case 



