126 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



In the other cases in which the sexual and asexual forms mor- 

 phologically resemble each other, as in Salpa, the origin of the 

 alternation of generations may, as in the case of the origin of the 

 direcious from the hermaphrodite state, be traced back to the ten- 

 dency towards the establishment of a division of labour acting upon 

 an animal which possessed the capacity of sexual and asexual repro- 

 duction. It was advantageous for the formation of the regular chain 

 of buds (stolo prolifer) that the power of sexual reproduction should 



be suppressed, and that the 

 generative organs should be- 

 come rudimentary and finally 

 vanish in the budding indivi- 

 duals ; while, on the other 

 hand, in the individuals 

 united in the chain, the gene- 

 rative organs were early de- 

 veloped, and the stolo prolifer 

 was aborted and completely 

 vanished. 



Special forms of alternation 

 of generations may be dis- 

 tinguished in which colonies 

 are formed as the result of 

 the asexual reproduction by 

 budding from a single animal, 

 the buds remaining attached 

 and developing into individuals 

 which differ considerably in 

 structure and appearance, and 

 each of which performs special 

 functions in maintaining the 

 colony (nutritive, protective, 

 sexual, etc.) Such a form of 

 alternation of generations is 

 known as polymorphism,* and reaches a great complication in the 

 polymorphous colonies of the Siphonophora. 



A form of reproduction which closely resembles metagenesis, but 

 which genetically has quite a different explanation, is the lately 



* K. Leuckart, " Ueber den Polymorphismus der Individuen oder die 

 Erscheinung der Arbeitstheilung in der Natur." Giessen, 1851. 



FIG. 113..;, Fully developed Strobila with sepa- 

 rating Ephyrse. h, Free Ephyra (of about 1 '5 to 

 2 rnm. diameter. 



