PROBLEM I . Why Offspring Resemble Their Parents 



end of the process, many in the in-be- 

 tween stages, and many that were not 

 dividino- at the moment the tissue was 

 killed. A careful search enables you to 

 put together the whole story told in the 

 preceding paragraph. And in so doing 

 you discover various structures that are 

 usually not seen in unstained cells. One 

 of these is the spindle, a structure of 

 threads extending from one end of the 

 cell to the other (see Fig. 406, 3-5). 

 The threads seem to support the chro- 

 mosomes. The chromosomes line up in 

 the middle of the spindle. Then after the 

 chromosomes have split lengthwise the 

 half chromosomes travel away from one 

 another. In cells which are near the end 

 of division, the new groups of chromo- 

 somes are at opposite ends of the spindle. 



The diagram of mitosis in an animal 

 cell shows other structures to be present. 

 While there are minor differences, mito- 

 sis in plants and animals is alike in that 

 each chromosome splits down the middle 

 after the genes have doubled. As a result 

 each new nucleus has the same number 

 of chromosomes and the same kinds of 

 genes as the original nucleus. The proc- 

 ess is easy to study in plant cells. Do 

 Exercise 3. 



What is accomplished by mitosis. 

 When a fertilized t^g divides, its nucleus 

 divides by mitosis. With continued divi- 

 sions of the same type, every cell of the 

 new individual will have the same kinds 

 of chromosomes and genes that the fer- 

 tilized t^^ had. This means that every cell 

 of the new individual has a full set of 

 genes just like those that were in the fer- 

 tilized e^^. These genes determine the 

 characteristics of the new organism. You 

 may wonder how one part of the body 



459 



forms an eye and another part a leg if all 

 the cells of both parts have the same 

 genes. This happens because in the place 

 where a leg is to form, only the "leg" 

 genes act; in the place where eyes form, 

 only the "eye" genes act. We do not 

 know with certainty why this is so but 

 this really important fact should not be 

 overlooked: a cat is a cat because the 

 genes in every one of its cells are "cat" 

 genes. Every cell has the same kinds and 

 number of genes as the fertilized egg; 

 and the fertilized to^g was formed when 

 an tg^ with cat genes and a sperm with 

 cat genes came together. The new indi- 

 vidual is therefore a cat and not some- 

 thing else. Its muscle is cat muscle, its 

 eyes have the characteristics of cat's 

 eyes, and so on. Every cell is a "cat" cell 

 because of the action of genes and be- 

 cause of the mitotic division of the chro- 

 mosomes with its genes that occurs 

 whenever cells divide. Note, too, that 

 this mitotic division gives every cat cell 

 the same number of cat chromosomes 

 and genes — no more and no less than the 

 fertilized t^g had. And so a certain num- 

 ber of chromosomes, 38 (19 pairs), be- 

 comes characteristic of the cat. Every 

 species of animal and plant has a definite 

 number of chromosomes. The table be- 

 low and Exercise 4 will interest you. 



