46 ZOOLOGY 



from the simple condition found in the gastrula (archenet- 

 ron). What is the fate of the blastopore? How does the 

 permanent mouth originate? 



63. Exercises for the Laboratory. 



The teacher should secure demonstrations of some of the 

 smaller ova (as of the snail, fish, sea-urchin) for examina- 

 tion with the microscope. Compare the ovum taken from the 

 ovary of a hen with a new laid egg, noting especially the struc- 

 ture of the latter. Obtain spermatozoa from the testis of a 

 recently killed animal (as mouse, fowl, etc.), and examine with 

 highest powers of the microscope. If possible secure permanent 

 mounts of segmenting eggs of sea-urchins, showing the 2-, 4- 

 8-celled stages. 



