Mammalia: Visceral Systems 



647 



-FUNDUS UTER 



NUCLEAR DIVISION 



FERTILIZATION COMPLETE 



IMPLANTATION 



SPERMATOZOON 



ST POLAR BODY STAGE 



UTERINE GLANDS' 



Fig. 489. Diagram illustrating the passage of the fertilized egg from the ovary 

 to the uterus. The human ovum is a week in the tube and three days in the uterus 

 before implantation takes place. (After Dickinson: "Sex Anatomy," Baltimore, 

 Williams & Wilkins Co.) 



adjacent Fallopian tube which, also by ciliary action, transmits the 

 egg back into the uterine region of the oviduct. The egg, fertilized in 

 the course of its passage along the Fallopian tube, becomes attached 

 to the wall of the uterus where, as it develops, it establishes a placenta 

 and the embryo enters upon a long period of gestation, at the end of 

 which an advanced stage of development and considerable size have 

 been attained. The uterine region is accordingly greatly enlarged and 

 its walls develop strong layers of "smooth" muscle which serve to 

 expel the young at birth. Posterior to their uterine regions, the right 

 and left oviducts of placental mammals always coalesce, the cavities 

 joining to form a single median chamber, the vagina. 



In most placental mammals the fusion of the posterior ends of the 

 two oviducts extends forward to involve also a greater or less extent of 

 their uterine regions, so that several types of uterus may be recognized 

 (Fig. 490). A uterus duplex results when the right and left uterine 

 regions remain apart, having separate openings into a common median 

 vagina (in some insectivores, most rodents, and conies, Hyrax). In 

 a two-horned uterus (uterus bicornis) the anterior regions of the two 

 uterine chambers remain apart, but their posterior regions join to form 

 a median space which has a single opening into the vagina (in cetaceans 

 and sea cows; in some members of each of the following groups — 

 insectivores, rodents, bats, carnivores, ungulates; and in lemurs). In a 

 uterus bipartitus the adjacent walls of the two uteri join to form a 

 more or less extensive median partition between the two cavities 

 which have a common opening into the vagina (in some bats; some 



