FROM SIMPLE CELL TO COMPLEX ANIMAL 47 



ber of chromosomes, formed by an animal, what is believed to 

 be the difference in result when eggs (which have a uniform 

 number) are fertilized by the two kinds ? 



63. Exercises for the Laboratory. 



, Specimens of some of the smaller ova (as of the snail, 

 fish, sea-urchin) may be secured for examination with the micro- 

 scope. Compare the ovum taken from the ovary of a hen with 

 a new laid egg, noting especially the structure 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 segment- 

 ing eggs of sea-urchins, showing the 2-, 4-, 8-celled stages, — up 

 to the gastrula. 



