20 INTRODUCTION TO THE EMBRYOLOGY OF THE LEOPARD FROG 



the fertilization of the egg and the appearance of the first cleavage 

 furrow. Often frogs are misled by an early thaw and proceed to shed 

 and fertilize their eggs, and then the pond freezes over. Such eggs 

 usually can withstand a brief (1 to 2 days) freezing without serious 

 effects. Once cleavage has begun, it must proceed (within certain 

 limits of speed) until the egg is divided into progressively smaller and 

 smaller units, first known as blastomeres and later as cells. The first 

 cleavages are quite regular. Since the egg has so much yolk the 

 division of parts of the egg becomes very irregular after about the 32- 

 cell stage. The cleavage planes in the early stages may be altered by 

 unequal pressures applied to any egg within a clump of eggs. 



The blastula develops an eccentric cavity because the animal pole 

 cells are so different from the large yolk cells of the vegetal hemisphere. 

 However, the end of the blastula stage is the end of the cleavage stage, 

 although cell division goes on throughout the life of the embryo, the 

 larva, and finally the frog. 



The gastrula is an embryo having two primary germ layers, the 

 epiblast (presumptive ectoderm and mesoderm) and the endoderm. 



Gastrulation in the frog. 



The second layer is continuous with the first and develops by integrated 

 movements of sheets of cells. There results the formation of a new 

 and second cavity known as the gastrocoel, or archenteron, which is 

 the primary embryonic gut cavity. The opening into this cavity is the 

 blastopore, and is located in the approximate region of the posterior 

 end of the gut cavity, or the region of the anus. 



The process of gastrulation in the frog is completed by providing 



