Hom AfWfials and Flants Reproduce unit viii 



ntal circulation 

 mbilical cord 



rion 



Fig. 383 In all jnammals ex- 

 cept a jew frovi Australia, 

 the yoimg develop within 

 the mother'' s body. The em- 

 bryo becomes attached to 

 certain membranes of the 

 uterus by a placenta. How 

 does the developing embryo 

 obtain food and oxygen and 

 get rid of waste products? 



Amniotic fluid 



Wall of uterus of parent 



Detail of 

 Placenta 



to develop. Their appetites are enormous, 

 and the parent birds make hundreds of 

 excursions every day in search of food 

 for them. 



When the young birds get a Httle 

 older they must learn to fly. The parents 

 often force the young to make their first 

 attempt by pushing them out of the nest. 

 But growth and development are slow. 

 Thus among some birds there is a type 

 of family life in which, usually, both par- 

 ents play an equal part. 



Reproduction in mammals. Fertiliza- 

 tion is internal in the mammals, even in 

 those few species, like the whale, that 

 spend their lives in water. We say that 

 their young are "born alive" but what 

 we mean is that the young develop 

 within the mother. A bird's egg when 

 laid is also very much alive but undevel- 

 oped. In practically all mammals the 

 young develop inside the body. But in- 

 ternal development occurs also at times 

 among plant lice and among some species 

 of snakes. Some fish are also born alive, 

 as vou know if you have ever raised 

 guppies. There is a difference, however, 



in the wav the mother among mammals 

 supplies food and oxygen to the develop- 

 ing embryo and removes the wastes from 

 it. You will learn about this presently. 

 In review^ do Exercise 10. 



How rabbits reproduce. Let us study 

 rabbit reproduction as an example of 

 reproduction in a mammal. Rabbits have 

 several litters of young in a year. At 

 certain times the ovaries release from 

 one to ten mature eggs. At this time the 

 male is attracted to the female and sperm 

 cells are placed in the lower part of the 

 oviduct. These travel up the oviduct. 

 Fertilization occurs in the upper portion 

 of the duct and the egg cell begins cleav- 

 age, going through much the same stages 

 — hollow ball, r^vo-layered cup, forma- 

 tion of three germ layers — that occur in 

 the development of the fish. Before de- 

 velopment progresses very far, the em- 

 bryo comes down to an expanded por- 

 tion of the oviduct, a sac called the 

 uterus (you'ter-us). In some mammals 

 the two oviducts unite, forming a single 

 uterus. The rabbit embryos, surrounded 

 by membranes, attach themselves to the 



