A kangaroo holding its baby in its pouch.

FIGURE 28–24 Marsupials Marsupial young, such as this wallaby peeking out of its mother's pouch, are born at a very early stage of development. They complete their development nursing in their mother's pouch.

▸ Marsupials Marsupials, such as the wallabies in Figure 28–24, bear live young that usually complete their development in an external pouch. Marsupial young are born at a very early stage of development. Little more than embryos, they crawl across their mother's fur and attach to a nipple in her pouch, or marsupium (mar SOO pee um). Inside the marsupium, the young spend months attached to a nipple. They continue drinking milk and growing inside the marsupium until they can survive independently.

▸ Placentals Placental mammals, such as the harp seals shown in Figure 28–25, are named for the placenta, which allows nutrients, oxygen, carbon dioxide, and other wastes to be exchanged between the embryo and the mother. The placenta allows the embryo to develop for a long time inside the mother and allows it to be born at a fairly advanced stage of development.

A harp seal with its offspring by its side in snow.

FIGURE 28–25 Placental Mammals Placental mammals, such as harp seals, are nourished through a placenta before they are born and by their mother's milk after they are born.

28.3 Assessment

Review Key Concepts

    1. Review Compare asexual reproduction and sexual reproduction in terms of the genetic diversity resulting from each.

    2. Infer Why might sexual reproduction, as opposed to asexual reproduction, produce a population better able to survive disease or environmental changes?

    1. Review Define the two types of fertilization.

    2. Predict Why would you expect species that employ external fertilization to reproduce in the water?

    1. Review Define the three ways in which embryos develop.

    2. Compare and Contrast What is the difference between a nymph and a pupa?

    1. Review What structure enables reptiles and birds to reproduce outside of water?

    2. Interpret Visuals In your own words, describe the functions of two of the membranes shown in Figure 28–22.

WRITE ABOUT SCIENCE

Creative Writing

  1. Write an advertisement for an amniotic egg. Draw and label the parts of the egg, including each of the membranes and the shell. Describe the purpose of each structure and how it's ideally suited for its function.

End ofPage 826

Table of Contents

Miller & Levine Biology
About the Authors
Changing the World of Biology Education
Contents
Diversity of Life: A Visual Guide DOL•1–DOL•64
Labs and Activities
Features
UNIT 1 The Nature of Life
1 The Science of Biology
2 The Chemistry of Life
Unit Project
UNIT 2 Ecology
3 The Biosphere
4 Ecosystems and Communities
5 Populations
6 Humans in the Biosphere
Unit Project
UNIT 3 Cells
7 Cell Structure and Function
8 Photosynthesis
9 Cellular Respiration and Fermentation
10 Cell Growth and Division
Unit Project
UNIT 4 Genetics
11 Introduction to Genetics
12 DNA
13 RNA and Protein Synthesis
14 Human Heredity
15 Genetic Engineering
Unit Project
UNIT 5 Evolution
16 Darwin's Theory of Evolution
17 Evolution of Populations
18 Classification
19 History of Life
Unit Project
UNIT 6 From Microorganisms to Plants
20 Viruses and Prokaryotes
21 Protists and Fungi
22 Introduction to Plants
23 Plant Structure and Function
24 Plant Reproduction and Response
Unit Project
UNIT 7 Animals
25 Introduction to Animals
26 Animal Evolution and Diversity
27 Animal Systems I
28 Animal Systems II
29 Animal Behavior
Unit Project
UNIT 8 The Human Body
30 Digestive and Excretory Systems
31 Nervous System
32 Skeletal, Muscular, and Integumentary Systems
33 Circulatory and Respiratory Systems
34 Endocrine and Reproductive Systems
35 Immune System and Disease
Unit Project
A Visual Guide to The Diversity of Life
HOW TO USE THIS GUIDE
THE TREE OF LIFE
Bacteria
Protists
Fungi
Plants
Animals
Appendices
Appendix A Science Skills
Appendix B Lab Skills
Appendix C Technology & Design
Appendix D Math Skills
Appendix E Periodic Table
Glossary
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q, R
S
T
U
V
W
X, Y, Z
Index
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
Credits
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