35? MANUAL OF ZOOLOGY SECT. 



between those of the other two. Each ovum is enclosed 

 in a follicle the Graafian follicle with a wall composed 

 of small cells. When the ovum approaches maturity the 

 follicle projects on the surface of the ovary, and eventually 

 the wall becomes ruptured and the ovum escapes into 

 the body-cavity. 



The oviducts, of which there are two, are not connected 

 with the ovaries, each opening anteriorly into the body- 

 cavity by a wide opening, except in the Dog-fish, in which 

 they unite anteriorly and have a single median opening. 

 In the Dog-fish (leaving this anterior connection out of 

 account) and in the Lizard the oviducts remain practically 

 distinct from one another throughout ; in the Rabbit 

 the posterior parts are united to form a median passage, 

 the vagina, leading to the exterior. The ova in all 

 three, when discharged from the ovaries, enter the wide 

 openings of the oviducts, and are impregnated during 

 their passage backwards. In both the Dog-fish and the 

 Lizard each fertilised ovum becomes enclosed while in 

 the oviduct in a tough shell, and is discharged when de- 

 velopment has only begun. In the Rabbit the fertilised 

 ovum is received into the posterior paired part of the 

 oviduct, or uterus, and there undergoes its development, the 

 young Rabbit when born differing little save in size from 

 the adult. The nourishment of the fcetus or uterine young 

 of the Rabbit is effected by means of a special vascular 

 structure known as the placenta, by the agency of which 

 nutrient material passes from the blood of the mother to 

 that of the foetus ; and after birth the young Rabbit 

 receives its nourishment for a time exclusively from the 

 secretion of a set of glands of the mother the mammary 

 or milk-glands. 





