426 



never goes through cleavage. This part 

 remains as an undivided mass of food or 

 as a "yolk sac" shown in A to E. Be- 

 cause of the yolk sac it is difficult to see 

 the hollow ball and cup stages of the em- 

 bryo, but they do form. 



Germ layers and tissues. The double- 

 walled cup (gastrula) develops a third 

 layer of cells, which lies between the 

 original two layers. There are now three 

 primary germ layers. The outer germ 

 layer is the ectoderm (ek'toe-derm), the 

 inner is the eiidoderin, and the middle 

 layer is the mesoder?n. The further de- 

 velopment of the three-layered cup is 

 complicated. It took years of study for 

 biologists to discover and describe the 

 many changes that occur. It would take 

 months of study to follow their descrip- 

 tions. But briefly this is what happens. 

 Cell division continues. The cup length- 

 ens and takes on the outlines of a fish, 

 lying somewhat curled up on top of the 

 yolk sac. By this time there are thou- 

 sands of cells. These change in various 

 ways and become the tissues, organs, 

 and organ systems of the baby fish. The 

 process by which the many different 

 kinds of tissue cells are produced is 

 called differentiation (diff'er-en-she-a'- 

 shun). 



The body covering and the nervous 

 system are formed from the ectoderm. 

 From the endoderm are fomied the lin- 

 ing of the alimentary canal and several 

 important organs, such as the liver, the 

 pancreas, the thyroid gland and the re- 

 spiratory system. The alimentary canal 

 develops slowlv, the mouth fomiing 

 from an opening that appears at the 

 closed end of the cup stage of the em- 

 bryo. From the mesoderm arc formct] 



Hoio A?ii?fials and Pkfits Reproduce unit viii 



the muscles, bones, blood vessels, the sex 

 glands, and some other body parts. What 

 you have just read about the formation 

 of primary germ layers and differentia- 

 tions occurs in all higher animals as well 

 as in the fish. 



Development is completed. During all 

 this time no food is taken in from the 



outside. Any slight increase in size is 

 caused by addition of water which en- 

 ters from the surroundings. The devel- 

 oping animal lives on the foods stored 

 in the original large egg cell. As the ani- 

 mal grows the yolk sac shrinks. By the 

 time the yolk sac is used up the baby 

 fish has organs for food getting and loco- 

 motion. When the tiny animal has or- 

 gans of digestion, breathing, locomotion, 

 and other organs that enable it to keep 

 alive by itself, the embr>^o stage has 

 ended. This development takes about 

 two weeks in some fish, a longer time in 

 others. See Exercise 5. 



How reproductive organs develop. You 

 have traced the development of a baby 

 fish from the uniting of the egg cell of 

 its mother and the spemi cell of its fa- 

 ther. But in time the baby animal will 

 itself become a parent; one generation 

 follows another. 



The reproductive organs do not de- 

 velop at the same time as the other or- 

 gans. This is what happens. Early in the 

 development of the embryo, when some 

 cells begin to lengthen into muscle cells 

 or change into any of the other tissue 

 cells (differentiation), a small group of 

 cells remains almost unchanged. Even 

 after the embryo has become a young 

 fish, these cells have not changed to any 

 extent; they have remained as undiffer- 

 entiated cells. 



