Chap. 19 DEVELOPMENT 367 



of thick ectoderm extends from the lip of the blastopore to the anterior end of 

 the body, the former animal pole. This is the neural plate from which the 

 nervous system is formed. 



At first, the roof of thearchenteron is flat. Gradually, three folds arise in 

 it and extend the length of the body. The central one of chordamesoderm be- 

 comes the notochord. Those on either side separate from the wall of the 

 archenteron and grow in between the ectoderm and endoderm (Fig. 19.5). A 

 cavity in each one will be part of the future coelom or body cavity. The outer 

 side of each fold adheres to the ectoderm and together they become the 

 somatopleure, the forerunner of the body wall; the inner side of each fold 

 unites with the endoderm to become splanchnopleure, the future wall of the 

 digestive canal. Mesodermal cells differentiate in these layers and form various 

 structures such as muscles. 



Development of the Frog 



Eggs. Small as they are, frogs' eggs are huge compared with those of 

 amphioxus and their bulk is largely yolk (Fig. 19.1, 19.6). As they float in 



Polar bodies 



Animal hemisphere 



Groy crescent 

 Vegetal hemisphere 

 Jelly coats (3) 

 ViteHine membrane 



Fig. 19.6. Frog's egg 35 minutes after fertilization. The protective jelly secreted 

 by the oviduct swells as soon as the eggs touch the water. The egg loses water dur- 

 ing the rearrangement of protoplasm that occurs at fertilization and the shrinkage 

 allows it to rotate within the fertilization membrane. The unfinished division re- 

 sulting in the second polar body has been stimulated to completion by the entrance 

 of the sperm. In one region the pigment has moved toward the entering sperm thus 

 creating the gray crescent. (Courtesy, Rugh: The Frog. Philadelphia, The Blakiston 

 Co., 1951.) 



