FOEMS OR REPRODUCTION. 



843 



polyzoa. In some composite animals (siphonophora) the different polyps perform different 

 functions. Some have a digestive, others a motor, and a third a generative function, so that 

 there is a physiological division of labour. Buds which are given off from the parent are formed 

 internally in the rhizopoda. In some animals (polyps, infusoria), which can reproduce them- 

 selves by buds or division, there is also the formation of male and female elements of generation, 

 so that they have a sexual and a non-sexual mode of reproduction. 



IV. Conjugation is a form of reproduction which leads up to the sexual form. It occurs in 

 the unicellular Gregarinae. The anterior end of one such organism unites with the posterior 

 end of another ; both become encysted, and form one passive spherical body. The conjoined 

 structures form an amorphous mass, from which numerous globular bodies are formed, and in 

 each of which numerous oblong structures the pseudo-navicelli are developed. These bodies 

 become, or give rise to an amoeboid structure, which forms a nucleus and an envelope, and 

 becomes transformed into a gregarina. 



Sexual reproduction requires the formation of the embryo from the conjunction of the male 

 and female reproductive elements, the sperm-cell and the germ-cell. These products may be 

 formed either in one individual (hermaphroditism, as in the flat worms and gasteropods), or in 

 tv)o separate organisms (male or female). Sexual reproduction embraces the following 

 varieties : 



V. Metamorphosis is that form of sexual reproduction in which the embryo from an early 

 period undergoes a series of marked changes of external form, e.g., the chrysalis stage, and the 

 pupa stage, and in none of these stages is reproduction possible. Lastly, the final sexually 

 developed form (the imago stage in butterflies) is pro- 

 duced, which forms the sexual products whose union 

 gives rise to organisms which repeat the same cycle 

 of changes. Metamorphosis occurs extensively 

 amongst the insects ; some of them have several 

 stages (holo-metabolic), and others have few stages 

 (henri-metabolic). It also occurs in some arthropoda, 

 and worms, e.g., trichina; the sexual form of the 



animal occurs in the in- 

 d \" testine, the numerous 



larvce wander into the 

 muscles, where they be- 

 come encysted, and form 

 undeveloped sexual or- 

 gans, constituting the 

 pupa stage of the muscu- 

 lar trichina. When the 

 encysted form is eaten by 

 another animal, the sexual 

 organs come into activity, 

 a new brood is formed, 

 and the cycle is re- 

 peated. Metamorphosis 

 also occurs in the frog 

 and in petromyzon. 

 [This is really a condition 

 in which the embryo un- 

 dergoes marked changes 

 of form before it becomes 

 sexually mature.] 



VI. Alternation of Generations (Steenstrup). In this variety some of the members of the 

 cycle can produce new beings non-sexually, while in the final stage reproduction is always sexual. 

 From a medical point of view, the life-history of the tape-worm or Taenia is most important. 

 The segments of the tape-worm are called proglottides (fig. 631), and each segment is herma- 

 phrodite, with testes, vas deferens, penis, ovary, &c, and numerous ova. The segments are 

 evacuated with the faeces. The eggs are fertilised after they are shed (fig. 626), and from them 

 is developed an elliptical embryo, provided with six hooklets, which is swallowed by another 

 animal, the host. These embryos bore their way into the tissues of the host, where they 

 undergo development, forming the encysted stage (Cysticercus (fig. 627), Coenurus, or Echino- 

 coccus (fig. 630). The encysted capsule may contain one (cysticercus) or many (coenurus) 

 sessile heads of the taenia. In order to undergo further development, the cysticercus must be 

 eaten alive by another animal, when the head or scolex fixes itself by the booklets and suckers 

 to the intestine of its new host (fig. 629), where it begins to bud and produce a series of new 

 segments between the head and the last-formed segment, and thus the cycle is repeated. 



The most important flat-worms are : Tsenia solium, in man ; the Cysticercus cellulosae (fig. 

 628), in the pig, when it constitutes the measle in pork ; Tnia mediocanellata (fig. 631), the 



Fig. 626. 

 A ripe egg taken from the 

 uterus of Taenia solium. 



a, Albuminous envelope; 



b, remains of the yelk ; 



c, covering of the embryo; 



d, embryo with em- 

 bryonal hooklets. 



Fig. 627. 

 Encapsuled cysticercus from Taenia 

 solium embedded in a human 

 sartorius. Natural size. 



