174 A GUIDE FOR THE STUDY OF ANIMALS 



Definitions. 



Cleavage stages, two, four, eight, sixteen cells, etc., aris- 

 ing by repeated division, starting with the egg. 



Blastula, a hollow sphere, the wall of which is composed 

 of a single layer of cells. 



Gastrula, a stage formed from the blastula by pushing 

 in one side of the latter, so as to form a more or less cup- 

 shape structure. 



Observations. 



1. Is there any considerable difference between the size 

 of the egg and the size of the blastula and gastrula .i" Has 

 development taken place by an increase of size or by an 

 increase of complexity .*' 



2. Contrast the blastula and gastrula as to number of 

 cavities, number of cell layers, ntimber of external 

 openings. 



3. Suggest protozoans that resemble the egg and blas- 

 tula respectively. What invertebrates resemble the gas- 

 trula in body plan .? 



Suggested drawings. 



a. Some of the cleavage stages. 



b. A blastula. 



c. A gastrula. 



3. Postembryonic Development or Metamorphosis of a Mosquito 



Materials. 



Some specimens of the larvae and pupae of the mos- 

 quito, ordinarily known as wrigglers. Either specimens 

 or diagrams of egg packets should also be provided. 

 Mounted specimens of adult of both sexes. 



