PROBLEM 1 Why Do Offspring Resemble Their Parents? 



Many animals have two different types 

 of cells. Let us examine again the de- 

 velopment of an animal such as the 

 fish. After fertilization the egg goes 

 through a series of rapid divisions. Grad- 

 ually the several kinds of tissue cells ap- 

 pear and the organs are formed of vari- 

 ous combinations of tissues. But certain 

 cells remain unchanged; they do not 

 form muscle, bone, nerve, or any of the 

 other tissues you have learned about. 

 Eventually, however, these cells go 

 through special cell divisions and develop 

 into gametes (eggs or sperms). A com- 

 plex animal, therefore, is really composed 

 of two main kinds of cells. There are the 

 tissue cells that make up the body; they 

 are called somatic cells and all their pro- 

 toplasm is called somatoplasm. The other 

 cells, fewer in number, are the reproduc- 

 tive cells. They are also called gervi cells, 

 and their total protoplasm is called gervt 

 plasm. The germ plasm in time forms 



gametes (sperms or egg cells). To test 

 your understanding do Exercise i. In 

 the higher animals this separation of 

 germ plasm and somatoplasm is quite dis- 

 tinct. The somatoplasm is not able to 

 produce another organism; only germ 

 plasm can do that. 



In plants and in lower animals, how- 

 ever, there may be no clear cut differ- 

 ence between germ plasm and somato- 

 plasm. In many plants any part of the 

 organism can produce a new plant of 

 the same kind. In some lower animals 

 after the reproductive organs have been 

 cut out, new ones may be made from 

 other tissues. But in the higher animals 

 there is a distinct difference between 

 cells that produce gametes, the germ 

 plasm, and cells that make up the rest of 

 the body, the somatoplasm. 



Cats produce more cats. You know- 

 that when cats reproduce, the offspring 

 are always cats; human beings produce 



Fig. 403 A diagrcmi to show continuity of germ plasm. Starting with the zygote 

 (left) trace the germ plasm (shown in black) to the body of the fish. In what organ 

 is it found in the adult fish? An egg from the fish, after fertilization, has fortned a 

 ball of cells (right). What will the ''black" cell in this ball become? 



