940 



VARIETIES OF GENERATION. 



one another (they may even possess a common "colonial" nervous system, like 

 the bryozoa) ; or they may detach themselves and individually enter upon an 

 independent existence. In some animal forms (siphonophores) , the individual 

 beings at times exhibit a definite differentiation of function, so that, for example, 

 digestive, motor and reproductive activities may be distinguished physiological 

 division of labor. The formation of buds within the organism, which subsequently 

 are detached, has been observed in rhizopods. Among animals that multiply 

 by division or by budding, there has been observed in part also the formation of 

 spermatozoa and ova (polyps, infusoria), so that here, together with asexual 

 reproduction, there is at the same time sexual reproduction. 



Conjugation or concrescence is the name applied to a variety of generation 

 that is already suggestive of sexual reproduction, for example that of unicellular 

 gregarines. In such a being the anterior extremity unites with the posterior 

 extremity of another. Both become encysted into one round, resting body. 

 The two nuclei coalesce and, after previous spindle-formation, a polar body 



351. Sexually Active (Middle) the Proglottis of Taenia mediocanellata (after Sommer): d, sexual eminence 

 with the genital pore e, into the latter the penis (cirrus, /) projects from above, advancing in the segment 

 into the tortuous vas deferens, which exhibits extensive ramification and leads to numerous testicular vesicles 

 (most of the testicular vesicles are not yet connected by excretory ducts with the vas deferens): g, vagina; h, 

 /ary; *, albumin-gland; k, group of shell-glands; /, uterus; b, excretory longitudinal trunk with transverse 

 anastomosis c; a, lateral nerve. 



is expelled. The united body-mass is resolved into a shapeless structure, from 



ich numerous vesicles arise. In each vesicle many boatlike figures appear 



(pseudonavicellae). These give rise to ameboid organisms, which by the forma- 



>i a nucleus and a protecting envelop are in turn transformed into gregarines. 



Concrescence has been observed also among some infusoria 



sexual reproduction requires the formation of the offspring from the union of 



male and the female generative elements (semen and ovum). These elements 



:rived from two different individuals, male and female, or they may 



long to the same individual (hermaphroditism, for example in tapeworms, 



snails, etc.) Sexual reproduction embraces also the following forms of generation : 



Metamorphosis is the name applied to that form of sexual reproduction in 



which, from the fertilized ovum on, the organism appears in a succession of out- 



itterent forms (for example caterpillar, chrysalis) , which possess no power 



