42 LABORATORY DIRECTIONS IN 



In a favorable specimen count the chromosomes in one cell. This num- 

 ber is the number of chromosomes found in all the body cells of Ascaris, 

 and is known as the somatic or diploid number. Recall the number of 

 chromosomes in the mature ovum (see 2e above), which is known as 

 the reduced or haploid number. The diploid number is restored at 

 fertilization. 



Compare maturation in the male and female germ cells (see chart). 



B. DEVELOPMENT 

 Development of the Frog 



The developmental processes of the various groups of vetebrates are 

 quite similar. The development of one of them, therefore, serves to 

 illustrate the process in all, just as the formation of spermatozoa and ova 

 in the grasshopper and Ascaris are typical of the corresponding processes 

 in other animals. 



Study the early development of the frog, using the following stages: 



1. During the First Day. 



la. Unsegmented Egg. Study with dissecting microscope, using 

 transmitted light and reflected light (the latter preferably with dark back- 

 ground). The middle of the black half is called the animal pole; the 

 middle of the white half the vegetative pole. Note the layers of jelly. 

 How many? Relative thickness? Draw an unsegmented egg, side view, 

 with the animal pole toward the top of the plate. Make the egg itself 

 % inch in diameter and the jelly in proportion. Label. 



16. Two cell stage. Note the cleavage furrow. Where is it deepest? 

 Draw the egg without the jelly, side view, and with the animal pole above. 

 Label the poles and furrow. 



Ic. Four cell stage. A second cleavage furrow is present. How is it 

 placed with respect to the first? Draw, without the jelly, inclining the 

 animal pole slightly toward you so as to show the intersection of the two 

 cleavage furrows. 



Id. Either twelve cell or sixteen cell stage. The cells of the animal 

 half of the egg divide more rapidly than those of the vegetative half, so 

 that there may be eight cells in the region of the animal pole while there 

 are only four near 'the vegetative pole. This would be a twelve cell 

 stage. The four vegetative cells soon divide, making sixteen in all. 



Note that the eight cells in the animal half are usually arranged roughly 

 in two rows of four cells each. Compare these cells in size with those 

 about the vegetative pole. Draw the twelve cell or sixteen cell stage, 

 inclining the animal pole toward you so as to show the entire group of 

 eight cells above, but still representing an oblique side view. Label the 

 animal and vegetative poles. 



