The male gametes of a mammal are of the usual vertebrate type, consisting of a head and a flagellum. The female gametes are just visible to the naked eye and contain practically no yolk, since the fertilized egg develops within the body of the female, being nourished with food and oxygen supplied by her blood stream. Fertilization is therefore internal and is brought about by a sexual act, when sperm from the male are transferred to the female by a special copulatory organ, the penis, through which runs a urogenital duct.
The two ovoid testes with their blood vessels and nerves originate near the kidneys and are connected, each by a sperm duct or vas deferens, with the urinogenital duct; later the testes pass down to occupy an externaljust posterior to the penis in scrotal sacs. In doing so each sperm duct loops over a ureter. Sperm pass from the testes to be stored in the uterus masculinus, whence they are shed during coition.
In the female the two ovaries retain their originaljust behind the kidneys. They are small and oval, having a somewhat blistered appearance in the adult animal, due to the fact that eggs are at various stages of development, the ripe ones breaking through the ovary wall and being shed at regular intervals. The funnel of each oviduct practically fits round its ovary, so that ripe eggs do not lie loose in the body cavity. They pass down the first part of each oviduct, the Fallopian tube, and come to rest attached to the wider part, the uterus. Coition usually occurs just before eggs are shed from the ovary, and the sperm swim up the uteri and one fuses with each egg there or, more commonly, in the Fallopian tubes. The eggs thereby fertilized proceed to develop rapidly, each young animal floating in fluid enclosed by membranes. In the later stages of development the embryo is known as afcelus and these membranes are called the fostal membranes. The region of attachment of the fcetal membranes to the wall of the uterus is called the placenta to which the fcetus is attached by the umbilical cord. The blood of the fcetus circulates not only around its body but also through the placenta via blood vessels running through the umbilical cord. In the placenta the blood streams of the fcetus and of the mother run
REGENERATION AND GROWTH side-by-side though the two do not actually mix, being separated by the thin walls of the blood vessels of the placenta. Food and oxygen in solution in the blood of the mother diffuse into that of the fcctus, while carbon dioxide and nitrogenous waste pass in the reverse direction.
The lungs, kidneys and alimentary canal of the foetus are not operative until after birth so that the foetus uses those of its mother indirectly. When all the organs of the fetus have developed birth occurs. The walls of the uterus contract, forcing the young rabbits to the exterior through the distended vagina and causing the fcetal membranes to burst. As each young animal emerges into the colder air, the shock causes it to gasp and to draw its first breath, thus inflating its lungs which then commence to function with the establishment of the breathing movements. The umbilical cord is ruptured leaving the fcetal membranes still inside the uterus. These are later expelled by further contractions of the uterus and emerge as the afterbirths. The end of the umbilical cord attached to the young rabbit soon drops off leaving a scar in the centre of the abdomen known as the navel or umbilicus. All the parts of the adult are now present, but they may not be fully developed, nor do many of them function until much later. If the eggs are not fertilized then they die and are shed.
It is characteristic of mammals that unlike the majority of other animals—birds and earwigs being notable exceptions—they look after their young until they are able to look after themselves,, protecting and teaching them, processes which reach their highest degree of complexity in man himself. The food of the very young animals is itself characteristic since mammals are animals possessing mamma which are ?w7/s-secreting glands and provide milk for the nourishment of the young until their teeth are sufficiently developed to enable them to eat solid food.