PROBLEM 2 . HouD More Co?nplex Anmials Reproduce 



Fig. 372 Develop?/?eTit of the 

 fertilized egg of a sii//ple 

 aiiii/ial. A, B, C, and D show 

 cleavage of the egg. E, F, G, 

 are sections of later stages. 

 What do they show? How 

 do you explain the very 

 slight increase in size? 



425 



Fig. 373 From fertilized egg, 

 through eii7bryo7iic stages, 

 to adidt fish. In E the ''''yolk 

 sac" is seen even after the 

 egg hatches. Of what use is 

 it to the growing aniinal? 

 Note the solid black spot in 

 D, E, F, and G. The cells 

 here reiTiain undifferentiated 

 at first, and later, in G, grow 

 into what organ? 



and the cell body immediately divide. 

 The nucleus begins the division. The 

 tw'o cells remain attached to one another 

 but do not grow larger as most cells do 

 after division. They keep on dividing 

 again and again vmtil after a few hours 

 there is a ball of smaller cells. Divisions 

 of the fertilized &^^ of a simpler animal 

 are illustrated in Figure 372. In the fish 

 and in all the higher vertebrates the 

 changes are similar but more difficult to 

 indicate in a diagram. The repeated di- 

 vision of the fertilized t^^ without 

 growth of the cells is called cleavage 

 (kleev'aj). Then the cells in the ball be- 

 come arranged into a larger hollow ball. 

 (The hollow ball is called a blastula — 



Beginning of 

 next generation 



blast'you-la.) Still floating in the water 

 the hollow ball of cells sinks in on one 

 side so that it is no longer spherical but 

 forms a double-walled cup (the gast- 

 nila — gast'roo-la). A diagram of this cup 

 is shown in F of Figure 372. From the 

 time of the first division and through 

 early development the organism is called 

 an embryo (em'bree-oh). This descrip- 

 tion of the development of an embryo 

 holds for all except the simplest of ani- 

 mals. It is possible to see all these stages 

 of development by examining prepared 

 slides. See Exercise 4. 



If you study next the figure that shows 

 the development of the fish Q^<g (Fig. 

 373) you will see that part of the t<g<g 



