REPRODUCTION AND DEVELOPMENT 47 



is the vegetal pole of the egg. Draw several eggs (X 3), showing their 

 envelopes of jelly as they lie together in the mass, and a single egg 

 from a side view ( X 20) to show the distribution of pigment and the 

 jelly envelopes. In this and in all subsequent drawings from the side 

 view, the animal pole should be toward the top of the page. Label 

 the poles and indicate the polar axis by an arrow drawn as though 

 thrust through the egg. 



C. Cleavage Stages 

 Exercise 45. — Collection and Study of Living Eggs. 



(a) It is desirable that cleavage as well as later stages in devel- 

 opment be studied in living eggs. The frogs and salamanders nor- 

 mally lay their eggs in spring and early summer, the period of 

 egg-laying being as definite a characteristic of the species as any 

 other form of behavior. If this study is made during the egg-laying 

 season, go out and collect eggs for yourself, examining the breeding 

 places and studying the activities of the animals under natural condi- 

 tions. Are there any easily recognizable differences between males 

 and females? Why do frogs croak more at this season? 



(b) It is possible to induce ovulation in a female frog in the late 

 fall and winter by inserting hypophyses from other frogs into her 

 coelom. The hypophysis, or so-called pituitary gland, may have been 

 seen lying in the cranial cavity and attached to the ventral surface 

 of the brain. When the secretion of these extra glands is carried to 

 the ovaries final growth of the egg cells occurs and ovulation follows. 

 Eggs thus obtained can be fertilized by adding sperm from a frog's 

 testis, and development will follow. 



(c) Place a mass of living eggs in a shallow dish and keep in a 

 well-lighted place but not exposed to direct sunlight for much of the 

 day. Record the stage of the eggs when obtained and note the changes 

 from day to day. Preserve your notes in the form of a written 

 report to be handed in later. The influence of temperature upon the 

 rate of development can be tested by placing part of the eggs out- 

 of-doors on the cool north side of a building and comparing them each 

 day with those having sunlight and the warmth of indoors. With 

 proper care development can be followed until the tadpoles have com- 

 pleted their metamorphosis. At no time should the water in the dish 

 be allowed to become too low from evaporation or to become foul 

 from the growth of bacteria. A few green water plants will be bene- 

 ficial. After hatching, the tadpoles can be fed upon bread or cracker 

 crumbs, but too much food will foul the water and care must be used. 



