304 REPRODUCTION IN ANIMALS 



body layers undergo many changes in attaining the adult 

 condition. The starfish, for example, forms two pouch- 

 like outgrowths on the tip of the intestine that finally 

 sever their connection, like bubbles blown from a pipe, 

 and ultimately form the body cavity, which contains the 

 digestive tract, and the germ-cells. 



At about the time the body cavity is developing, the 

 mouth cavity appears as a depression in the ectoderm 

 opposite the free end of the gut with which it finally 

 comes in contact. The walls at the point of union 

 become perforated and the alimentary canal thus becomes 

 a tube open at both ends. 



Vertebrate Development. — The development of the 

 starfish, as thus described, is in a broad way essentially 

 the same as that occurring in most of the larger groups of 

 animals. Beyond this stage the growth of the individual 

 is largely a matter of detail without profoundly modify- 

 ing the fundamental plan of the body. The frog, for 

 example, passes through a gastrula stage, develops a body 

 cavity, and thenceforth travels a developmental path that 

 is characteristically vertebrate. The most striking feature 

 of this early period of growth is the development of the 

 central nervous system, which in the late gastrula stage 

 makes its appearance as two folds, close together, that 

 extend the length of the animal along the back (Fig. 73). 

 These increase in height, meet and fuse along the mid- 

 line, and the tube thus formed separates from the skin 

 and sinks beneath it. 



The mouth cavity that has formed, as in the starfish, 

 develops in all classes of vertebrates four or five gill-slits 

 that finally open to the exterior, as in a fish. In later life 

 the gills disappear in the air-breathing vertebrates. The 

 digestive tract lengthens, forms a stomach, and develops 

 pouches that finally result in liver, pancreas, and, except 

 in the fishes, lungs. Organs of special sense are in large 

 measure modifications of the ectoderm and are of too 

 complicated a character to permit of a brief description. 



