542 ON HEREDITY. 



spoudiug cbanges which occur in the protophism of the egg which siir- 

 roiiucls it, other cells arise by a process of division, and these in their 

 turn also multiply by division. These cells arrange themselves in course 

 of time into layers, which are termed the germinal or embryonic layers. 

 From these layers arise all the tissues and organs of the body, both in 

 its embryonic and adult stages of life. 



The starting-point of each individual organism — i. e., of each new gen- 

 eration — is therefore the segmentation nucleus. Every cell in the adult 

 body is derived by descent from that nucleus through repeated division. 

 As the segmentation nucleus is formed by the fusion of material derived 

 from both parents, a physical continuity is established between par- 

 ents and offspring. But this physical continuity carries with it certain 

 properties which cause the offspring to reproduce, not only the bodily 

 configuration of the parent, but other characters. In the case of man 

 we find along with the family likeness in form and features, a corre- 

 spondence in temperament and disposition, in the habits and mode of 

 life, and sometimes in the tendency to particular diseases. This trans- 

 mission of characters from parent to offspring is summarized in the 

 well-known expression that " like begets like," and it rests upon a phys- 

 ical basis. The size of the particles which are derived from the parents 

 (called the male and female i)ronuclei), the potentiality of which is so 

 utterly out of proportion to their bulk, is almost inconceivably small 

 when compared with the magnitude of the adult body. And yet these 

 particles are sufficient to stamp the characters of the parents, of the 

 grandparents, and of still more remote ancestors on the offspring, and 

 to preserve them throughout life, notwithstanding the constant changes 

 to which the cells forming the tissues and organs of the body are sub- 

 jected in connection with their use and nutrition. 



In considering the question of how new individuals are produced, one 

 must keep in mind that it is not every cell in the body which can act 

 as a center of reproduction for a new generation, but that certain cells, 

 which we name germ-cells and sperm-cells, are set aside for that pur- 

 pose. These cells, destined for the production of the next generation, 

 form but a small proportion of the body of the animal in which they 

 are situated. They are as a rule marked off" from the rest of the cells or 

 of its body at an early period of development. The exact stage at which 

 they become specially differentiated for reproductive purposes varies 

 however in different organisms. In some organisms (as is said by Bal- 

 biani to be the case in Chironomtis) they apparently become isolated 

 before the formation of the germinal layers is completed ; but as a rule 

 their appearance is later; and in the his/her organisms, not until the 

 development of the body is relatively much more advanced. 



The germ-cells after their isolation take no part in the growth of the 

 organism in which they arise; and their chief association with the other 

 cells of its body is that certain of the latter are of service in their nutri- 

 tion. The problem therefore for consideration is the mode in which 



