THE PHYSICAL BASIS OF HEREDITY. 



37 



the parents, a fact of which advantage is taken to preserve the peculiar 

 characters of our varieties of fruit, all of which are perpetuated by 

 grafting, or by means of runners. 



In some worms the power of developing a new individual if part 

 of the old one has been lost has been modified so that the lost parts are 

 reformed before they are really lost. A given limited part of the mid- 

 dle of the body has the habit of forming a new head for the part behind, 

 and a new tail for the part in front. A string of individuals is formed 

 in this way joined tandem. These separate after some time and each 

 new individual repeats the process. 



Pig. 5. FIG 6 



Fig. 5. Hydras: the One at c with Buds; the One at d with Sex-Organs. From 

 Leuckart. 



Fig. 6. An Arm of a Starfish reproducing Four Lost Arms. 



These methods of forming new individuals are occasional. Each 

 method is restricted to some limited group of species. In practically 

 all animals in which these occasional methods of reproduction occur, 

 they alternate with sexual reproduction. In the great majority of 

 animals sexual reproduction is the only means of transmitting char- 

 acters to a new generation. 



By sexual reproduction we understand the development of a new 

 individual from a single cell which is usually produced by the fusion of 

 two cells. 



Just a word as to Avhat we mean by a cell. The word has a peni- 

 tentiary flavor that may be misleading. A cell is a mass of protoplasm 

 enclosing a differentiated portion or nucleus. The nucleus contains, 

 among other things, during certain phases of cell life a definite 

 number of thread-like bodies called chromosomes. The number of 

 chromosomes differs in different animals, but is always the same in the 

 different cells of the same animal. 



