BIRDS. 341 



renal portal system. Characteristic of birds is an arterial plexus 

 beneath the skin of the ventral side, which becomes greatly 

 enlarged at the time of incubation. The red blood corpuscles 

 are oval and nucleated. 



The permanent kidneys are metanephridia. Usually they 

 consist of three lobes, each lobe lying in a cavity bounded by 

 the vertebrae and the transverse processes. Frequently the 

 kidneys meet and even fuse posteriorly. The ureters open 

 separately in the cloaca. No urinary bladder occurs. The 

 urine is white and semi-solid. 



The left ovary is never functional, and it and its duct are 

 usually aborted. The right ovary is strongly lobulated on ac- 

 count of the large size of the eggs. The corresponding ovi- 

 duct has a large funnel-shaped opening, and is divided into 

 three regions, the middle of which is glandular and furnishes 

 the white, while the posterior is both muscular and glandular 

 and secretes the egg-shell. From the fact that the egg remains 

 some time in the latter division, this is sometimes spoken of as 

 the uterus. The testes are usually equally developed ; they lie 

 in front of the kidneys, and the vasa deferentia have a convo- 

 luted course, opening separately into the cloaca. A copulatory 

 organ is usually absent. In the ostrich there is a solid retrac- 

 tile penis like that of alligators and turtles, while a few other 

 ratites and aquatic birds have the ventral wall of the cloaca 

 thickened, with a median groove which serves as a sperm duct. 

 Secondary sexual characters are common. The male may be 

 either larger or (more rarely) smaller than the female. Fre- 

 quently he is distinguished by brighter colors, by the develop- 

 ment of certain feathers, etc. 



The eggs are very large, and are incubated by the parents ; 

 the period of incubation varying from eleven days to seven 

 weeks (ostrich). The nest-building habits vary greatly. 



The development of different birds shows few and unim- 

 portant variations, and the history of the common chick is well 

 known. The early phases of segmentation are passed through 

 before the egg is laid. This segmentation affects at first only 

 a small portion of the upper surface of the yolk (i.e., is mero- 

 blastic). The resulting blastoderm is several cells in thickness, 



