EARLY CLEAVAGES 113 



In the New York area Rana sylvatica breeds in March. After the ice has 

 melted in small woodland ponds and there have been a few days of warm, 

 rainy weather, the males and females come out of hibernation and migrate 

 to the edge of the pond. They tend to congregate at a certain region within 

 the pond, and year after year the descendants return to the same place. The 

 males are somewhat smaller than the females. The male mounts the female 

 and his forelimbs clasp the upper part of the abdomen very firmly. Clasping 

 pairs may remain in this relationship for many hours. 



The female deposits her eggs in the water in a single mass which is held 

 together by a jelly. The sperm are shed by the male, and fertilization takes 

 place within a matter of minutes. Shortly after fertilization a membrane 

 swells off the surface of the egg and the egg then floats freely within the jelly. 

 Because the vegetal (white) hemisphere is heavier than the animal (black) 

 hemisphere, the eggs rotate within the jelly until the animal pole is upper- 

 most. If the mass of fertilized eggs is turned upside down, each egg will 

 rotate back to its original position. The orientation of the egg with respect 

 to gravity appears to be important, because in some species of frogs develop- 

 ment becomes abnormal if the egg is upside down for a definite period of 

 time during first cleavage. Double monsters are produced by inverting eggs 

 at the time when a cleavage occurs. 



Early cleavages 



Cleavage and subsequent development to the early larval stage proceed 

 within the jelly and membrane. The jelly swells to many times its original 

 volume by taking up water. Swelling separates the eggs from one another, 

 and thus a more favorable environment is provided for each egg within the 

 mass. Finally at hatching the membrane and some of the jelly is attacked by 

 an enzyme secreted by the embryo, and the membrane breaks. The embryo 

 emerges from the jelly and soon after begins to swim. The description that 

 follows is for the most part limited to the egg of Rana sylvatica. For the 

 purpose of illustrating certain details, however, a photograph of the egg 

 of a salamander, Amby stoma opacum, has been included. 



Figure 55 shows the changes from fertilization (stage number l) to the 

 16-cell stage (number 6) . The jelly and membrane are omitted. This period 

 takes about five hours at 18° C. At fertilization the sperm enters the animal 

 hemisphere and about one hour later the gray crescent becomes visible on the 



