THE CELL. 41 



Often spores are formed which differ from the parent cell, but the 

 spores are evidently asexual, neither being produced by sexual or- 

 gans nor presenting sexual characteristics. 



The varieties of asexual reproduction are the following : 

 (a) Normal fission, in which a unicellular animal simply divides 

 into two ; example, bacteria and amoeba?. 



(6) Budding. In this the unicellular organism produces a bud 

 wlncl. eventually becomes cut off, forming a new individual; ex- 

 ample, yeast. 



(c) By endospores.In this method new cells are formed with 

 in a large cell; example, lichens. 



(d) Rejuvenescence. By this process the protoplasm assumes a 

 rounded mass, escapes from the cell-wall, and forms for itself a 

 new cell-wall; examples, spirogyras and diatoms. 



(e) Spore-reproduction. The spore is a modified cell, whose 

 function is to perpetuate and reproduce the species. Spores are 

 generally formed in a spore case, or sporangium. The structural 

 elements of the spore are the exosporium,, or outer coat; endospo- 

 rium, or inner coat ; and the protoplasm. . . 



(2) Sexual reproduction. This occurs when one or two sexual 

 cells engage to reproduce a plant or animal. Among plants the 

 sexual elements are the spermatazoids and the oospheres. The 

 sexual elements of animals are the spermatazoa and the ova. The 

 varieties of sexual reproduction are conjugation, parthenogenesis, 

 and fertilization. 



(a) Conjugation. This consists in the union of two like cells. 

 In some cases the protoplasm of one cell is discharged into that of 

 another, the resulting cell being called a zygospore, or auxospore; 

 example, Spirogyra. It may occur, also, by the union of the proto- 

 plasm of two distinct cells which have previously discarded their 

 cell-walls. In this method the cells are structurally the same, but 

 as the process is analagous to that of the sexual method that is, 

 cytoplasm fusing with cytoplasm, and nucleus with nucleus, it is 

 considered by the best authorities as sexual in character ; examples, 

 diatoms and animalcules. 



(#) Parthenogenesis. This occurs when one sex alone pro- 

 duces a new individual. A single sexual cell may be concerned or 



