CHORDATA 





ureter, which opens into the dorsal side of the urinary bladder ; 

 from the urinary bladder a single duct, the urethra, passes to 

 the outside. 



The male reproductive organs consist of the two testes, which 

 have variable positions in the different mammals ; in some, such 

 as the whales, they lie in the abdominal cavity in the region of 

 the kidneys ; in some, such as the elephants, they pass back- 

 ward to the region of the urinary bladder ; the latter condition 

 is found also in rodents, bats, and some others except during the 

 breeding season, and then they pass into temporary pockets 

 formed by evaginations of the ventral abdominal wall and called 

 the scrotal sacs ; in almost all other mammals they lie perma- 

 nently in the scrotal sacs and cannot normally pass back into 

 the abdominal cavity ; the two scrotal sacs may remain distinct 

 or they may fuse into one, the scrotum, but the internal cavities 

 remain distinct. Many ducts pass off from each testis, uniting 

 in a single vas deferens for each, which opens into the urethra 

 near the bladder. The urethra receives also the ducts from the 

 prostate and Cowper's glands, whose secretions are mingled with 

 the spermatozoa at the time of ejaculation. The urethra is 

 inclosed in an erectile penis. The two ovaries lie near the pos- 

 terior portion of the abdominal cavity ; when the ova are mature 

 they pass into the oviducts, or Fallopian tubes as they are called 

 in the Mammalia, and these lead into the uterus ; there may be 

 two uteri or the two may be fused into one ; the uterus opens 

 into the vagina, which leads to the outside. One group of mam- 

 mals is oviparous, the group to which the duckbill belongs, lay- 

 ing eggs about two centimeters in diameter ; but all the rest are 

 viviparous, and the ova, which are extremely small, practically 

 microscopic, develop within the uterus. 



In the course of development an amnion and an allantois are 

 formed as in reptiles and birds, and there is almost always an 

 intimate connection between these membranes and the wall oi 

 the uterus. The membranes are connected with the ventral 

 body wall of the embryo or fetus by means of a stalk, the um- 

 bilical cord, through which blood vessels pass, and the branches 

 of these blood vessels in the membranes come into close contact 

 with the maternal blood vessels of the wall of the uterus ; thus 

 by osmosis the nutritive fluids pass from the mother to the 



