180 PRINCIPLES OF ANIMAL BIOLOGY 



is well known cither as a regular or occasional occurrence. Some plants 

 as wheat and beans regularly self-fertilize. Other plants as the violet 

 produce some flow(M"s which are regularly cross-fertilizcKl and others which 

 can only be self-fertilized. Among parasitic flatworms (the cestodes and 

 trematodes) both cross- and self-fertilization have been observed. In- 

 deed, in some species of tapeworms which possess two sets each of male 

 and female organs in each proglottis, every conceivable form of cross- 

 and self-fertilization has been observed. 



Combined Types of Reproduction. — In the foregoing pages enough 

 has been stated in regard to the reproductive processes to show that 

 many animals reproduce by more than one general method. In many 

 (probably most) life histories which include budding, fission, or sporula- 

 tion there is always at some time also sexual reproduction. Likewise in 

 most animals which employ parthenogenesis there is at times a period of 

 bisexual reproduction as illustrated by the life history of many species 

 of aphids. In some instances there is no known regularity in the recur- 

 rence of the sexual act, and in many cases there is no striking difference 

 in the structure of the asexual and sexual individuals. In the type of 

 life cycle known as metagenesis, however, there is a regular alternation 

 between asexual and sexual reproduction accompanied by definite changes 

 in the form of the individual. The Hydrozoa, of which Obelia and 

 Bougainvillea are examples, illustrate this kind of life history. In 

 Obelia the colony arises from a single polyp by a method of budding 

 (asexual). Two types of buds are produced, the hydranth or nutritive 

 polyp, and the blastostyle which together with the enveloping sheath 

 forms the gonangium. By 'budding the blastostyle produces individuals 

 of a third type, the medusce, which vipon maturity are released to swim 

 freely in the water. The medusae which are similar to those of Bougain- 

 villea (Fig. 62) are sexual individuals producing eggs or sperms. Upon 

 fertilization the egg develops into a ciliated embryo, the planula, which 

 soon attaches itself and develops into a polyp from which by budding 

 a colony arises. The polyps are incapable of sexual reproduction and 

 the medusae of most species are incapable of asexual reproduction. The 

 sexual and asexual forms are unlike in structure. Metagenesis is rare 

 outside the Coelenterata but what appears to be metagenesis has been 

 reported in a few species of marine annelid worms and in some other groups. 

 In the worms, however, the difference in structure between sexual individ- 

 uals and the vegetative individuals is not so great as in the Hjdrozoa. 



Artificial Parthenogenesis. — The exact nature of sexual reproduction 

 has been a matter of speculation and in recent years a matter of pains- 

 taking research. The researches of Loeb and others upon artificial 

 stimulation of development of the egg have thrown considerable light 

 upon the effect of the sperm upon the egg. Development of the egg 

 induced by artificial means is called artificial parthenogenesis. 



