Descriptive Biology 101 



former which is known as the "sub-neural gland/' and has 

 been compared to the vertebrate hypophysis, which is part 

 of the pituitary body or gland attached to the base of the 

 brain, one of those problematic organs of * ' internal secretion ' ' 

 which is playing so large a part in medicine today. The 

 larva swims actively by means of its long tail, but at metamor- 

 phosis the latter is lost together with its supporting rod or 

 notochord, and the animal abandons the wandering ways of 

 youth and settles down to its future monotonous existence. 



In those tunicates in which an alternation of generations 

 occurs, the asexual form gives rise by budding to a colony 

 of "zooids" which more or less directly produce the sexual 

 animals. 



The meaning of this strange and interesting life history 

 remains for the future to disclose. Until we understand the 

 underlying significance of sex, it is hopeless to attempt to 

 solve the riddle of alternation of generations. In our attempt 

 to find an answer to both of these two great questions of 

 biology, the experimental, rather than the purely morpho- 

 logical method gives the greater promise of success. There 

 is already at hand evidence to show that the appearance of 

 sexual reproduction can be to a certain extent at least con- 

 trolled by experiment. Details regarding such experiments 

 may be postponed however to a later chapter. 



The most likely interpretation of alternation of genera- 

 tions however is that it is a ''device" of Nature to increase 

 the spread of the species and thus enhance its chances of 

 survival. It occurs typically in attached forms such as most 

 plants, and the hydroids, Polyzoa and some tunicates among 

 animals, while in those marine worms in which it occurs the 

 asexual generation often lives a retired and inactive life in 

 the recesses of some rocky shelter, only the sexual generation 

 swimming freely at the surface of the sea. It obviously 

 increases the chances of distribution of a fixed form to have 

 a sexual free-swimming form which may carry the repro- 

 ductive cells and scatter them far and wide. A difficulty in 

 such an interpretation however is that in some of these forms 

 the sexual generation is not free-swimming but attached to 

 the asexual one. This may of course be a degenerate con- 

 dition. 



In the case of the plants on the other hand, it is the asexual 

 form with its numerous spores, which is most important in 

 the distribution of the species, these spores being readily 

 carried by the wind and thus better suited to spreading a 

 land form, than the flagella-bearing sperms, which require 

 water for their distribution. 



Among the Protozoa alternation of generations frequently 



