58 THE FROG 



extend over a period which varies in length with different species. 

 Note the changes in the shape of the body, in the mouth, the tail, 

 the legs, and the coloration. What becomes of the operculum? Have 

 the tympanic membranes appeared? Draw representative individuals. 



I. Development in Related Forms 



Exercise 67. — The Tadpole of a Salamander. 



(a) During the spring months the living tadpoles of the salamander 

 Amhlystoma may be obtained. Watch individuals as they swim in an 

 aquarium and then examine them in a watch glass with a handlens. 

 Compare, part by part, with the tadpoles of the frog already studied. 

 The suckers are long stalklike structures ; otherwise the resemblance is 

 obvious. Add a drop of ether or chloretone to the water, and when 

 the larvse become quiet examine with the low-power objective of the 

 compound microscope. Note the pigmentation. Observe the circula- 

 tion in the capillaries of the gills or tail. Can you recognize the blood 

 cells? Is there a pulse? 



(b) Older larva3 are often taken during the early summer in places 

 where frog tadpoles are abundant. Examine preserved specimens, 

 about 40 mm. in length, showing external gills attached to the oper- 

 culum. Compare with demonstration specimens of the adult. Can 

 you recognize mesodermal somites in larva and adult? What struc- 

 tures justify the statement that a salamander is a less specialized 

 vertebrate than a frog? Draw the larva from a side view, orienting 

 the same as the figures of the frog tadpoles. 



Exercise 68. — The Unincubated Egg of the Hen. 



(c) Take an unincubated hen's egg and, using scissors, cut open on 

 one side a space about 25 mm. across, being careful that the points 

 of the scissors do not injure the yolk. The opening may be further 

 enlarged, if necessary, while the egg is resting upon cotton in a finger 

 bowl. The familiar yolk and white of the egg will be observed. Find 

 the chalazae, or twisted cords of albumen, at each end. What relation 

 have they to the yolk and to the shell? Find the two membranes that 

 line the shell. These can always be seen at the larger end of the egg 

 where there is an air space between them. At one place upon the sur- 

 face of the yolk is a small whitish area, the blastoderm, the central 

 part of which is known as the area pellucida and the peripheral part 

 as the area opaca. Docs this always appear at the top, however the 

 egg is turned? Compare with the rotation of the frog's egg in its 

 jelly envelopes. Compare such an egg with that of the frog. Under- 



