. Adventures with animals and plants. Biology. PROBLEM Hoiv More Coviplex Ammals Reproduce 433 In this way the rabbit embryo is nour- ished and continues growing for about thirty days. At the end of this period the muscular walls of the uterus con- tract violently, the contractions causinu the young animal together with the pla- centa to be detached from the uterus. Through further contractions the em- bryo and the placenta are pushed through the vagina (va-jeye'na), the passage which leads from the uterus to the outside of the body. In all mammals development and birth occur in the same w
. Adventures with animals and plants. Biology. PROBLEM Hoiv More Coviplex Ammals Reproduce 433 In this way the rabbit embryo is nour- ished and continues growing for about thirty days. At the end of this period the muscular walls of the uterus con- tract violently, the contractions causinu the young animal together with the pla- centa to be detached from the uterus. Through further contractions the em- bryo and the placenta are pushed through the vagina (va-jeye'na), the passage which leads from the uterus to the outside of the body. In all mammals development and birth occur in the same way. But the period of internal develop- ment, called gestation (jes-tay'shun) varies in length. In the dog it is longer, about 9 weeks. In the cow it is a little over 9 months; in humans about 9 months; in elephants, almost two years. Now do Exercise ii. Care of the young. The amount and kind of care given by mammals varies with the species, but all feed the young on milk produced in mammary glands. Milk is not secreted by the mother until after birth of the young. You may know that farmers cannot expect to milk a cow until after the cow has a calf. Kangaroo and opossum babies are very immature when born. The mother immediately puts them into a pouch in- side of which are located the mammary glands. There the young receive warmth, protection, and a constant supply of food. Among few mammals are the young so immature when born. But in many, such as the dog, cat, mouse, guinea pig, and rabbit, the young are born blind and are almost helpless. When the hu- man baby is born, its eyes are open, but it is entirely helpless and dependent upon the care of parents. On the other hand, Fig. 384 The Wallaby is one kind uf kangaroo. (new YORK ZOOLOGICAL PARK) walls of the uterus. The region of attach- ment, called the placenta (pla-sen'ta), is formed partly from the membranes of the embryo and partly from the lining of the uterus. If several embryos develop at the same time, which is what c
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