PROBLEM 2. 



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 com- 

 monly happens in rabbits, each usually 

 has its own placenta. A cord grows out 

 of the embryo to the placenta. Very 

 early in the embryo's development a 

 simple heart and blood vessels form. Sev- 

 eral large blood vessels run from the em- 

 bryo through the cord. They end in 

 capillaries in the placenta, and the blood 

 is returned by large vessels through the 

 cord. The capillaries in the placenta lie 

 close to other capillaries which are part 

 of the mother's circulatory system. In the 

 placenta food and oxygen diffuse from 

 the mother's blood through the capillary 

 walls into the embryo's blood stream. 

 Likewise, carbon dioxide and other 

 wastes diffuse out of the embryo's blood 

 into the parent's blood. Blood from the 

 parent never flows directly into the em- 

 bryo; there is no direct connection be- 

 tween the two circulatory systems. 



