6 HEREDITARY CHARACTERS 



fertilisation, however, a fresh series of simple divisions is 

 started. In the multicellular organism, all the tissues of the 

 body are built up by cells produced by a process of division 

 from a single cell, without any further fertilisation. Among 

 the cells that form the body of an individual is a particular 

 group, the members of which group are thrown off, as 

 described above, from the body. When they are thrown off, 

 these cells are capable of fusing with another cell thrown off 

 from a similar individual in a similar manner. This fusion 

 inaugurates a fresh cycle of simple divisions, which results 

 in the production of a new and independent body. Accord- 

 ing to this theory, the body of the multicellular organism is 

 practically a colony of unicellular organisms living together 

 and dependent upon each other ; and with certain limitations 

 this idea is sufficiently near the truth to be very helpful in 

 obtaining a proper idea of the nature of a multicellular 

 organism and of the transmission of characters from parents 

 to offspring. 



As all the cells forming the multicellular body are 

 derived from a single cell, this single cell produces all the 

 different kinds of cells that go to the formation of the animal 

 or vegetable body. 



In the case of unicellular organisms, any given kind of 

 organism produces organisms that are like itself. We find 

 in fact that cells, when in the form of unicellular organisms, 

 produce other cells like themselves. On the other hand 

 in the case of the multicellular organism, the single cell 

 the fertilised ovum produces at first cells that are like 

 itself, but later on, though still within a comparatively few 

 generations, it produces a great variety of cells, including 

 individuals that are quite as unlike each other as different 

 species of unicellular organisms. These different kinds of 

 cells are always produced in the same order ; that is to say, 

 the same series of different kinds of cells are always pro- 

 duced at the same periods in the building up of the organism 

 from the fertilised ovum . Thus the fertilised ovum of one 

 species of animal produces similar groups of cells to the 



