436 How Afiimals and Plants Reproduce unit viii 



male? In what part of the body do they lie? The spemi ducts are very 

 small so you may not find them. Through them the sperm cells enter the 

 kidney, whence they are carried to the exterior. How do the egg-produc- 

 ing organs (ovaries) of the female differ from the spermaries of the male? 

 Can you find ducts which might be oviducts? How many are there? 

 Note that the oviducts are larger than the sperm ducts. Study some of the 

 eggs from the ovary. Note the color. Estimate the number of eggs in one 

 ovary. 



3. What is the structure of a sperm? Crush the spermary of a freshly 

 killed frog (or the seminal vesicle of an earthworm) under water. Ex- 

 amine a drop under high power. Describe what you see. Examine a pre- 

 pared slide of mouse or rat sperm. In what ways are the sperms alike? 



4. What are some of the early stages in the development of the ferti- 

 lized egg? Use prepared slides showing the early development of a star- 

 fish or sea urchin. Find and draw one, two, four, and many-celled cleav- 

 age stages. How do they compare in size with the fertilized egg cell? 

 Explain. Find a clear section of a hollow-ball stage; of a cup stage. Draw. 

 How many layers of cells does the wall of the cup (gastrula) contain? 



5. Demojistratioii. Development of the fish. Using a museum prepara- 

 tion or chart study the changes which occur as the fertilized tgg develops 

 into an embryo. Examine the tgg closely for the first indication of the 

 formation of an embryo. At this stage has the tgg changed in size or 

 shape? As the embryo grows larger, what happens to the tgg} 



6. Deinoiistration. Development of a tadpole into a frog. In the spring 

 gather frogs' eggs and watch their development. (Water must be changed 

 frequently.) At other seasons, examine museum preparations or charts. 

 How does the tadpole feed? Describe the changes in its feeding and loco- 

 motion as it grows older. Which legs appear first? Look for the legs just 

 before they break through. How does the tail disappear? Explain. 



7. How do the reproductive organs of the hen differ from those of the 

 female frog? Examine the organs taken from a chicken prepared for cook- 

 ing. Do you find eggs? How many? W^hat size are they? What indication 

 is there that the eggs are not all produced at one time? Is the same thing 

 true of the frog or fish? How many ovaries and oviducts do you find? 

 Where must the white of the tgg and the shell come from? Where must 

 the tgg be \\hen it is fertilized? How do you know? 



8. What is the structure of a hen's tgg? Crack a raw tgg into a saucer. 

 What do you find directlv under the shell? How many membranes do 

 you find? Examine the shell and the membranes at the blunt end. What 

 do you discover? The yolk is the egg cell. What makes it so large? Do 

 you notice a tiny white spot on the yolk? This is the protoplasm and 

 nucleus which receive the sperm cell. Of what use to the growing embryo 

 is the white of tgg} The shell? The membrane? Compare with starfish or 

 sea urchin tgg (prepared slide), or frog or fish egg from a dissected ani- 

 mal. How does it differ? When eggs, like birds' eggs, have heavy cover- 

 ings, what do you know about the type of fertilization of that animal? 



