254 ESSENTIALS OF ZOOLOGY 



In its most primitive phase the placenta is a union of serosa 

 and uterine epithelium, and it always starts as such ; but 

 in placental mammals the mucous and submucous layers of 

 the uterus are largely replaced by the plasmodium of the 

 serosa. In rare cases the egg fails to enter the oviduct, and 

 after fertilisation grows and establishes its placenta on the 

 peritoneum. 



Birth takes place by contractions of the muscular walls 

 of the uterus, the bursting of the amnion and the serosa. The 

 umbilical or navel connexion of the foetus with the serosa 

 and the amnion breaks, or, in cases where help is given, is cut 

 through. The envelopes are detached completely from the 

 uterus as the afterbirth. 



The length of gestation is very constant in the different 

 species. It is 30 days in the rabbit, 20 days in the mouse, 

 56 days in the cat, 62 days in the dog, 840 days in the horse, 

 280 days in the cow, 140 days in the sheep, and 113 days in 

 the pig. It is probable that a hormone secreted by the foetus 

 gives the signal for birth, but attempts have been made to 

 show that the periodicity is connected with the periodicity 

 of ovulation. The ovary is certainly concerned in the uterine 

 life, for when the ovum is discharged the follicle changes into 

 a corpus luteum if fertilisation takes place. Its growth and 

 persistence may be due to endocrine relationship between the 

 ovary and the uterus, and its presence stimulates the mammary 

 glands to come into function. 1 



