INHERITANCE 101 



seems definite enough. However, it is really much 

 more complicated than appears at first sight, and 

 it will be well to consider briefly certain preliminary 

 matters before dealing with the more serious 

 attempts that have been made to grapple with its 

 difficulties. 



In the first place it should be remembered that 

 reproduction, whether of animals or plants, is 

 effected in two principal ways ; asexual and 

 sexual ; and that the phenomena of inheritance are 

 seen in both cases. Thus, to take a familiar case, 

 the common fresh-water Hydra reproduces either 

 by budding, or by the formation of eggs. The 

 former is an asexual process, the bud appearing as 

 a hollow outgrowth from the body wall of the 

 parent, which acquires mouth and tentacles at its 

 distal end, and after a longer or shorter time 

 detaches itself and becomes an independent Hydra. 

 An egg, on the other hand, is a specialised cell of 

 the ectoderm, or outer layer of cells of the parent, 

 which is incapable of development of any sort until 

 it has been fertilised by a spermatozoon, from the 

 same or another animal. After fertilisation the 

 egg segments, i.e., divides repeatedly so as to give 

 rise to a number of cells from which, by further 

 growth and differentiation, an embryo and ulti- 

 mately an adult Hydra is produced. The two 

 modes of reproduction, sexual and asexual, are 

 absolutely unlike, and yet the final results are the 

 same ; for so far as we are aware, there are no 

 points of difference that will distinguish with 

 certainty a Hydra produced by budding from one 



