THE BIOLOGICAL BACKGROUND OF GENETICS 197 



of reproduction, but they may all be classified into two main categories: 

 somatogenic and cytogenic. Somatogenic reproduction is accomplished 

 without the instrumentality of sex and involves the subdivision of the 

 parent body into two or many fragments, each of which has the power 

 to reconstitute a whole new individual like the parent. In unicellular 

 organisms somatogenic reproduction involves the division of the one- 

 celled body into two or more cells each capable of growing into a full- 

 sized individual; in multicellular organisms the products of division 

 are multicellular fragments. Cytogenic reproduction is accomplished 

 by means of unicellular germ cells which must pass through processes 

 of growth and division in order to reconstitute the multicellular body 

 typical of the species to which they belong. 



The essential feature of all reproductive processes is that some por- 

 tion of the parent is isolated, or physically cut off, from the parent's 

 body, an integral part of which it formerly was. On ceasing to be a 

 part of an organized system, it exercises its prerogative of developing 

 a system of its own in the image of the one from which it was derived. 

 This is true whether the isolated part be a single cell or a multicellular 

 fragment. This is perhaps to be expected in view of the fact that 

 each cell possesses the hereditary organization characteristic of the 

 race or species. 



Varieties of somatogenic reproduction. — In unicellular organisms 

 the principal modes of sexless reproduction are transverse fission, in 

 which the division is across the long axis of the cell; longitudinal fis- 

 sion, in which the division is parallel with the long axis; and multiple 

 fission, in which the nucleus divides several times before the cytoplasm 

 partitions the several nuclei off to form separate cells. In multicellular 

 organisms the principle modes of somatogenic reproduction corre- 

 spond rather closely to those among unicellulars, but several special 

 modes are added. Transverse fission is common among worms of vari- 

 ous kinds and among the larvae of certain jelly fishes. Longitudinal 

 fission takes place in the early embryos of a number of mammals, as in 

 armadillos and in man, resulting in true quadruplets and true twins 

 derived from an original single embryo. Multiple fission occurs in a 

 number of parasitic insects, in which the multicellular embryo sub- 

 divides into hundreds of cell-masses each of which becomes a complete 

 individual. In addition to these types of fission, multicellular or- 

 ganisms reproduce by budding, which may be simple or multiple. 

 Uudding involves the outgrowth of a minor portion of the parent 

 body, the latter body being left intact — this being in contrast with 



