PROCESS OF REPRODUCTION IN ORGANISMS. 2/ 



physiologically old. And here in order that anything further 

 may occur the stimulus of fertilization is necessary. 



This type of life cycle is probably connected with the great 

 development and specialization of vegetative organs and vegeta- 

 tive reproduction in the plant. The sporophyte becomes so large 

 and so highly differentiated that parts of it are physiologically or 

 physically isolated before the stage of gamete formation is 

 reached. These parts retain some degree of specialization during 

 their regulation after isolation and so produce the gametophyte, 

 which is usually much simpler morphologically than the sporo- 

 phyte, but is physiologically specialized in the direction of 

 gamete formation. 



The occurrence of apogamy in the gametophytes of some species 

 merely shows that less highly differentiated cells are capable of 

 producing asexually the same result which in the more highly 

 differentiated gametes occurs only after fertilization. 



Alternation of generations, so-called, or more properly meta- 

 genesis, in animals is different in certain respects from alternation 

 of generations in plants. In animals the asexual cycle always 

 occurs before maturation and maturation and fertilization are 

 never separated by a developmental cycle. Moreover, in ani- 

 mals the asexual bud which gives rise to the so-called sexual 

 generation usually undergoes its earlier development as a 

 specialized part of the asexual colony and becomes free if at all 

 only when its development is advanced. Its formation is, 

 however, undoubtedly connected with advancing differentiation 

 and senescence in the asexual colony or in that part of it from 

 which the sexual generation arises: in the hydroids, for example, 

 the medusa bud very commonly arises from the most highly 

 differentiated part of the asexual form, viz., the hydranth. It 

 is probable, moreover, that it is initiated by the physiological 

 isolation of a part as in other similar types of reproduction. In 

 this respect the phenomena of metagenesis are essentially similar 

 to those of alternation of generations in plants. In both cases 

 there is a second reproductive and developmental cycle inter- 

 polated between the first and the formation of new gametes: in 

 the plants this cycle occurs after maturation, in the animals 

 before. 



