264 STRUCTURE OF THE VERTEBRATES 



pass through the cervical opening into the uteri and fertilize 

 the ova in their anterior ends. 



The vaginal septum disappears in the placentals. However, in 

 the more primitive orders (insectivores and rodents) the two 

 uteri are distinct and each has a cervical opening into the 

 vagina. This is a duplex uterus. Further fusion of the two ducts 

 forms a bipartite uterus (found in carnivores and certain fami- 

 lies of other orders) with a single cervical opening, but the two 

 uteri united only at their posterior ends. The bicornuate uterus 

 occurs in horses, sheep, cows, and most other herbivorous ani- 

 mals, and is functionally a single uterus. The majority of 

 animals of this type have only one embryo developing at a time 

 and it occupies a median position in the uterus. Displacement 

 occurs when twins develop. ]\Iammals with duplex or bipartite 

 uteri more frequently have a number of offspring at a birth. 



The simplex uterus is a further step in the evolution of the 

 organ, and is characteristic of the anthropoids and man. The 

 uterus has a single median cavity, with the small oviducts 

 (Fallopian tubes of human anatomy) passing from the lateral 

 ovaries to the uterus. The uterus is roughly triangular in shape, 

 the apex being at the cervix. 



]\Iale. In the monotremes the seminal fluid containing the 

 spermatozoa from the testes is thrown into a common urinogeni- 

 tal sinus and conducted through the grooved penis. The latter 

 lies within the cloaca, but is protruded during coitus. 



Two paired erectile bodies (the cavernous bodies) are added to 

 the penis in the marsupials, and the lateral walls of the groove 

 grow medially to form a tube. The tube is in the unpaired 

 median body of the penis, and is functionally a continuation of 

 the urethra. A cross section of the organ appears as a single 

 ventral body containing the urethra, and paired erectile bodies. 

 The penis with its muscles is attached to the ischial bones, and 

 distally is enclosed within a sheath of cutaneous tissue. The 

 urethra opens through the glans, an enlargement of the median 

 body. 



The position of the penis shifts in the upper groups of mam- 

 mals. In the monotremes it lies within the cloaca and closes 

 the anal opening as it protrudes. With the disappearance of the 



